orinoco: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200225011415.GA31868@embeddedor

drivers/net/wireless/intersil/orinoco/fw.c

@@ -49,7 +49,7 @@ struct orinoco_fw_header {
 	__le32 pdr_offset;      /* Offset to PDR data from eof header */
 	__le32 pri_offset;      /* Offset to primary plug data */
 	__le32 compat_offset;   /* Offset to compatibility data*/
-	char signature[0];      /* FW signature length headersize-20 */
+	char signature[];      /* FW signature length headersize-20 */
 } __packed;
 
 /* Check the range of various header entries. Return a pointer to a

drivers/net/wireless/intersil/orinoco/hermes.h

@@ -341,7 +341,7 @@ struct agere_ext_scan_info {
 	__le64	timestamp;
 	__le16	beacon_interval;
 	__le16	capabilities;
-	u8	data[0];
+	u8	data[];
 } __packed;
 
 #define HERMES_LINKSTATUS_NOT_CONNECTED   (0x0000)

drivers/net/wireless/intersil/orinoco/hermes_dld.c

@@ -64,7 +64,7 @@
 struct dblock {
 	__le32 addr;		/* adapter address where to write the block */
 	__le16 len;		/* length of the data only, in bytes */
-	char data[0];		/* data to be written */
+	char data[];		/* data to be written */
 } __packed;
 
 /*
@@ -76,7 +76,7 @@ struct pdr {
 	__le32 id;		/* record ID */
 	__le32 addr;		/* adapter address where to write the data */
 	__le32 len;		/* expected length of the data, in bytes */
-	char next[0];		/* next PDR starts here */
+	char next[];		/* next PDR starts here */
 } __packed;
 
 /*
@@ -87,7 +87,7 @@ struct pdr {
 struct pdi {
 	__le16 len;		/* length of ID and data, in words */
 	__le16 id;		/* record ID */
-	char data[0];		/* plug data */
+	char data[];		/* plug data */
 } __packed;
 
 /*** FW data block access functions ***/

drivers/net/wireless/intersil/orinoco/orinoco_usb.c

@@ -202,7 +202,7 @@ struct ezusb_packet {
 	__le16 crc;		/* CRC up to here */
 	__le16 hermes_len;
 	__le16 hermes_rid;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /* Table of devices that work or may work with this driver */