mmc_spi: add a status check for spi_sync_locked

In case spi_sync_locked fails, the fix reports the error and
returns the error code upstream.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>