b44: Truncate PHY address

Some ROMs on embedded devices store incorrect values for
the PHY address of the ethernet device.
It looks like the number is sign-extended.
Truncate the value by applying the PHY-address mask to it.
The patch was tested on a bcm47xx embedded system (where the bug
triggers) and a bcm4400 PCI card.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>