wl12xx: AP mode - record TX configuration settings

Record TX configuration settings in the "conf" member of our global
structure (struct wl1271) if conf_tx is called when the firmware is
not loaded.

Later on when the firmware is loaded, we apply the tx conf as part of
the init sequence.

Important for AP mode since conf_tx is called before add_interface
(where the firmware is initialized).

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