b43: Kconfig: let config B43_BCMA_PIO depends on B43

Logically, config B43_BCMA_PIO should depend on B43. This also solves
the problem that sub options of b43 driver didn't indent correctly in
make menuconfig's ncurses window.

Signed-off-by: Zhao, Gang <gamerh2o@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>