wl18xx: allow setting supported mcs rates

The supported mcs rates are set using the rx_mask field of the
ieee80211_mcs_info structure.
Add an rx_mask module param that can be used for overriding the default
rx_mask value.

[Arik - formatting changes]

Signed-off-by: Eyal Reizer <eyalr@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>