b44: always set duplex mode why phy changes

Without this patch b44_check_phy() was called when the phy called the
adjust callback. This method only change the mac duplex mode when the
carrier was off. When the phy changed the duplex mode after the carrier
was on the mac was not changed. This happened when an external phy was
used.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>