mmc: dw_mmc-pltfm: fix the potential NULL pointer dereference

platform_get_resource can be returned the NULL pointer.
Then regs->start should be referred to NULL Pointer.
devm_ioremap_resource() checks whether res is NULL or not.

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