mmc: dw_mmc: NULL dereference in error message

The "host->dms->ch" pointer is NULL here so we can't use it to print the
error message.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>