mmc: sdhci-esdhc-imx: don't exit in case of no pinctrl states

The commit ad93220d ("mmc: sdhci-esdhc-imx: change pinctrl state
according to uhs mode") exits the probe in case there are no valid
pinctrl states found.

As there are configurations doing the pin mux properly in the boot
loader, don't exit. Just warn, but go on in case if there are no
pinctrl states in the device tree.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Andrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>