[POWERPC] More DCR native fixups

Getting BenH's new EMAC driver working on 440GP, I found some more
problems in the native mode paths of the new DCR code:
	- dcr_map() is supposed to return a dcr_host_t, but the native
version is a macro that doesn't expand to an expression.  With native
DCRs, dcr_host_t is an empty structure, so we just use a constructor
expression instead.
	- dcr_unmap() uses {} instead of the safer do {} while (0)
idiom to implement a no-op

Here's a fix.
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/dcr-native.h

@@ -26,8 +26,8 @@ typedef struct {} dcr_host_t;
 
 #define DCR_MAP_OK(host)	(1)
 
-#define dcr_map(dev, dcr_n, dcr_c)	{}
-#define dcr_unmap(host, dcr_n, dcr_c)	{}
+#define dcr_map(dev, dcr_n, dcr_c)	((dcr_host_t){})
+#define dcr_unmap(host, dcr_n, dcr_c)	do {} while (0)
 #define dcr_read(host, dcr_n)		mfdcr(dcr_n)
 #define dcr_write(host, dcr_n, value)	mtdcr(dcr_n, value)