isdn: hardware: mISDN: Remove reference to CONFIG_8xx

CONFIG_8xx is deprecated and should soon be removed in favor
of CONFIG_PPC_8xx.
Anyway, hfc_multi_8xx.h only uses 8xx I/O ports which are
linked to the CPM1 communication processor included in the 8xx
rather than the 8xx itself.

This patch therefore makes it dependent on CONFIG_CPM1 instead,
like several other drivers.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>