b43: Remove PIO support

Remove b43 PIO support.
DMA works well on all supported devices. There's no reason to use PIO.
Additionally, new devices don't support PIO in hardware anymore.
b43 PIO support is dead and unused code.

After applying this patch please do
git rm drivers/net/wireless/b43/pio.h
git rm drivers/net/wireless/b43/pio.c
to remove the main PIO support code.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>