drivers/net/fec_mxc.c: Fix compile warning

Fix this:
fec_mxc.c: In function 'fec_probe':
fec_mxc.c:926:13: warning: assignment from incompatible pointer type

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>