wl12xx: fix non-wl12xx build scenarios

Support building wl1271-equipped boards without building the
wl1271 driver itself, e.g.:

CONFIG_MACH_OMAP_ZOOM3=y
CONFIG_WL12XX is not set

Reported-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>