wl1271: make ref_clock configurable by board

The wl1271 device is using a reference clock that may change
between board to board.

Make the ref_clock parameter configurable by board settings
instead of having a hard coded value in the sources.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>