wl1271: Workaround for reference clock setting on boot.

If the 38.4MHz reference clock is configured to the firmware, it crashes
on boot. Configuring an experimental 38.4MHz in XTAL mode allows the
firmware to boot, and everything appears to work.

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