[SERIAL]: Avoid 'statement with no effect' warnings.

When SUPPORT_SYSRQ is false, gcc can emit warnings for
the uart_handle_sysrq_char() that results.  Using an
empty inline returning zero kills the warning.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/serial_core.h

@@ -385,11 +385,11 @@ int uart_resume_port(struct uart_driver *reg, struct uart_port *port);
 /*
  * The following are helper functions for the low level drivers.
  */
-#ifdef SUPPORT_SYSRQ
 static inline int
 uart_handle_sysrq_char(struct uart_port *port, unsigned int ch,
 		       struct pt_regs *regs)
 {
+#ifdef SUPPORT_SYSRQ
 	if (port->sysrq) {
 		if (ch && time_before(jiffies, port->sysrq)) {
 			handle_sysrq(ch, regs, NULL);
@@ -398,11 +398,9 @@ uart_handle_sysrq_char(struct uart_port *port, unsigned int ch,
 		}
 		port->sysrq = 0;
 	}
+#endif
 	return 0;
 }
-#else
-#define uart_handle_sysrq_char(port,ch,regs)	(0)
-#endif
 
 /*
  * We do the SysRQ and SAK checking like this...