wl1271: Add pre-power-on sleep

This patch adds a short delay before powering on the wl1271. Normally, it is
not needed, but if the wl1271 has been powered off shortly before, for reliable
firmware-booting this small stabilization delay is required.

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>