wl1271: Support FEM autodetection

In the NVS/INI-parameters for the driver/firmware there is the FEM
(front end module) selection option, which may configure the hardware FEM
via autodetection or manual selection. So far, there has been support for
manual selection only.

Add support for FEM autodetection.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>