[PATCH] airo: simpler calling convention for enable_MAC()

Callers of enable_MAC() shouldn't have to worry about the bits in the
response's status word (and most of them don't). The return value is
sufficient information.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>