drivers/net/armada100_fec.c: Fix compile warning

Fix this:
armada100_fec.c: In function 'armada100_fec_register':
armada100_fec.c:723:12: warning: assignment from incompatible pointer type

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