slip: fix macro redefine warning

MIPS/IA64 define END as assembly function ending, which conflict
with END definition in slip.h, just undef it at first

Reported-by: lkp@intel.com
Signed-off-by: Huang Pei <huangpei@loongson.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/slip/slip.h

@@ -40,6 +40,8 @@
 					   insmod -oslip_maxdev=nnn	*/
 #define SL_MTU		296		/* 296; I am used to 600- FvK	*/
 
+/* some arch define END as assembly function ending, just undef it */
+#undef	END
 /* SLIP protocol characters. */
 #define END             0300		/* indicates end of frame	*/
 #define ESC             0333		/* indicates byte stuffing	*/