wl12xx: AP mode - fetch appropriate firmware for AP

AP and STA modes use different firmwares.

Differentiate the firmware files by name and fetch the appropriate one
when add_interface is called by mac80211. The STA firmware is chosen for
PLT mode.

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