net: macb: fix mapping of registers

Some architectures (MIPS) needs mapping to access IOMEM.
Fix that.

Fixes: f1dcc19b ("net: macb: Convert to driver model")

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>