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 <rdunlap@infradead.org>
Cc: Chas Williams <3chas3@gmail.com>
Cc: linux-atm-general@lists.sourceforge.net
Cc: netdev@vger.kernel.org
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/atm/nicstar.c

@@ -91,7 +91,7 @@
 #ifdef GENERAL_DEBUG
 #define PRINTK(args...) printk(args)
 #else
-#define PRINTK(args...)
+#define PRINTK(args...) do {} while (0)
 #endif /* GENERAL_DEBUG */
 
 #ifdef EXTRA_DEBUG