[PATCH] fec_8xx: make CONFIG_FEC_8XX depend on CONFIG_8xx

Change CONFIG_FEC_8XX to depend on CONFIG_8xx instead of CONFIG_FEC.
CONFIG_FEC depends on ColdFire CPUs, which does not apply for the
PPC 8xx processors.

Signed-off-by: John W. Linville <linville@tuxdriver.com>