drivers/net/macb.c: Fix compile warning

Fix this:
macb.c: In function 'macb_eth_initialize':
macb.c:564:15: warning: assignment from incompatible pointer type

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