drivers/net/at91_emac.c: Fix compile warning

Fix this:
at91_emac.c: In function 'at91emac_register':
at91_emac.c:524:12: warning: assignment from incompatible pointer type

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