net: bcmgenet: test MPD_EN when resuming

When the GENET driver resumes from deep sleep the UMAC_CMD
register may not be accessible and therefore should not be
accessed from bcmgenet_wol_power_up_cfg() if the GENET has
been reset.

This commit adds a check of the MPD_EN flag when Wake on
Magic Packet is enabled. A clear flag indicates that the
GENET hardware must have been reset so the remainder of the
hardware programming is bypassed.

Fixes: 1a1d5106 ("net: bcmgenet: move clk_wol management to bcmgenet_wol")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>