atm: nicstar: fix if-statement empty body warning

When debugging via PRINTK() is not enabled, make the PRINTK()
macro be an empty do-while block.

Thix fixes a gcc warning when -Wextra is set:
../drivers/atm/nicstar.c:1819:23: warning: suggest braces around empty body in an ‘else’ statement [-Wempty-body]

I have verified that there is no object code change (with gcc 7.5.0).

Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Chas Williams <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: David S. Miller <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/atm/nicstar.c

···
 #ifdef GENERAL_DEBUG
 #define PRINTK(args...) printk(args)
 #else
-#define PRINTK(args...)
+#define PRINTK(args...) do {} while (0)
 #endif /* GENERAL_DEBUG */
 
 #ifdef EXTRA_DEBUG