wl12xx: Validate FEM index from ini file and FW

Check for out of bound FEM index to prevent reading beyond ini
memory end.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Cc: stable@kernel.org
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>