staging: fsl-dpaa2/eth: Refactor MAC address setup

The driver logic for allocating a MAC address to a net device
is complicated enough to deserve a function of its own. While
here, cleanup a bit the code comments.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>