mwl8k: add the AP version of USE_FIXED_RATE

As with the STA version, unicast will use auto rate adaptation, but
the AP version allows setting the rates to be used for management and
multicast transmissions, which can be set based on the BSS basic rate
set.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>