b43: don't duplicate common PHY read/write ops

Most of the PHYs use the same way of accessing registers, so move that
code to the shared place. An exception is G-PHY which sometimes access
A-PHY regs and requires special handling.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>