bgmac: fix "cmdcfg" calls for promisc and loopback modes

The last (bool) parameter in bgmac_cmdcfg_maskset says if the write
should be made, even if value didn't change. Currently driver doesn't
match the specs about (not) forcing some changes. This makes it follow
them.

Reported-by: Nathan Hintz <nlhintz@hotmail.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>