wl12xx: AP-mode - overhaul rate policy configuration

Use the minimal rate configured in the basic rates set as the AP
broadcast and multicast rate. The minimal rate is used to ensure weak
links can still communicate.

When the basic rates contains at least one OFDM rate, configure all
unicast TX rates to OFDM only.

Unify rate configuration on initialization and on change notification
into a single function.

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