b44: add calls to phy_{start,stop}

When support for external phys was added to b44, the calls to start and
stop the phy were missing in the mac driver. This adds the calls to
phy_start() and phy_stop().

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>