can: ucan: Replace zero-length array with DECLARE_FLEX_ARRAY() helper

Zero-length arrays are deprecated and we are moving towards adopting
C99 flexible-array members, instead. So, replace zero-length arrays
declarations in anonymous union with the new DECLARE_FLEX_ARRAY()
helper macro.

This helper allows for flexible-array members in unions.

Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/214
Link: https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.htmlSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/YzIdHDdz30BH4SAv@workSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/ucan.c

@@ -245,7 +245,8 @@ struct ucan_message_in {
 		/* CAN transmission complete
 		 * (type == UCAN_IN_TX_COMPLETE)
 		 */
-		struct ucan_tx_complete_entry_t can_tx_complete_msg[0];
+		DECLARE_FLEX_ARRAY(struct ucan_tx_complete_entry_t,
+				   can_tx_complete_msg);
 	} __aligned(0x4) msg;
 } __packed __aligned(0x4);