mmc: dw_mmc: remove unused EVENT_XFER_ERROR

EVENT_XFER_ERROR isn't been used now, so it can be removed.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>