b43: Completely remove support for phy_a

Per Michael Büsch: "All a-phy code is usused", so remove it all.

Cc: Michael Büsch <m@bues.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>