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>