mmc: dw_mmc: fix error return code in dw_mci_probe()

Fix to return -ENOMEM in alloc workqueue error case instead
of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>