net: bcmgenet: properly disable password matching

bcmgenet_set_wol() correctly sets MPD_PW_EN when a password is specified
to match magic packets against, however, when we switch from a
password-matching to a matching without password we would leave this bit
turned on, and GENET would only match magic packets with passwords.

This can be reproduced using the following sequence:

ethtool -s eth0 wol g
ethtool -s eth0 wol s sopass 00:11:22:33:44:55
ethtool -s eth0 wol g

The simple fix is to clear the MPD_PWD_EN bit when WAKE_MAGICSECURE is
not set.

Fixes: c51de7f3976b ("net: bcmgenet: add Wake-on-LAN support code")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>