net: fec_mpc52xx: don't discard const from netdev->dev_addr

Recent changes made netdev->dev_addr const, and it's passed
directly to mpc52xx_fec_set_paddr().

Similar problem exists on the probe patch, the driver needs
to call eth_hw_addr_set().

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: adeef3e3 ("net: constify netdev->dev_addr")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>