net: isdn: Make isdn_ppp_mp_discard and isdn_ppp_mp_reassembly static

Fix sparse warnings:

drivers/isdn/i4l/isdn_ppp.c:1891:16: warning:
 symbol 'isdn_ppp_mp_discard' was not declared. Should it be static?
drivers/isdn/i4l/isdn_ppp.c:1903:6: warning:
 symbol 'isdn_ppp_mp_reassembly' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>