wlcore/wl12xx: add hw op to get rate-mask for AP-link in STA mode

In some chip-families, there are operating modes where we must mask-out
certain Tx rates, and/or tweak the rate-mask with special HW-specific
bits.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>