wl12xx: Add mutex protection for interface list

The interface list maintained in main.c is not mutex protected. This could
cause issues, as the list is accessed from notifier chains.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>