b43legacy: reinit on too many PHY TX errors

Restart the hardware on too many PHY TX errors. A thousand PHY TX errors
per 15 seconds means we won't be able to recover for sure.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>