b43: Add mask/set capability to debugfs MMIO interface

This adds an atomic mask/set capability to the debugfs MMIO interface.
This is needed to support mask and/or set operations from the userspace
debugging tools.

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