net: dpaa: Use mac_dev variable in dpaa_netdev_init

There are several references to mac_dev in dpaa_netdev_init. Make things a
bit more concise by adding a local variable for it.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Acked-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>