Add a HW op to add extra enabled rates for AP-mode data-rates. Since
the rates might depend on channel properties, reconfigure AP-mode rates
when these change.

Implement the HW op for the 18xx family, where MIMO or wide-chan rates
can be added.
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>