mmc: mxs-mmc: remove unneeded NULL check

Static checkers complain about the inconsistent NULL checking here:

	drivers/mmc/host/mxs-mmc.c:680 mxs_mmc_probe()
	warn: variable dereferenced before check 'ssp->dmach' (see line 660)

The variable can't actually be NULL so we can remove the check.

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