mfd: cros_ec: move locking into cros_ec_cmd_xfer

Now that there's a central cros_ec_cmd_xfer(), move the locking
out of the SPI driver.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>