b43: use agent R/W ops for BCMA_IOCTL

BCMA_IOCTL is register in agent (AKA wrapper) core, we need to use
special R/W ops for it.

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