IB/nes: Fix a compiler warning

Avoid that the following compiler warning is reported when building with
W=1:

drivers/infiniband/hw/nes/nes_hw.c:646:51: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/nes/nes.h

@@ -159,7 +159,7 @@ do { \
 
 #define NES_EVENT_TIMEOUT   1200000
 #else
-#define nes_debug(level, fmt, args...)
+#define nes_debug(level, fmt, args...) do {} while (0)
 #define assert(expr)          do {} while (0)
 
 #define NES_EVENT_TIMEOUT   100000