mmc: dwmmc: Change designware MMC 'clksel' callback function to return status

Change 'clksel' callback function to allow the code to return a
status.

This patch is a preparation for enabling Arm-Trusted-Firmware (ATF)
in Intel SoC FPGA. This patch does not change functionality.

When using Arm-Trusted-Firmware (ATF) in Intel SoC FPGA, the MMC clock
related register is secure register which is required to be written
via SMC/PCSI call. It is possible that U-Boot fail to write the
register if there is unexpected error between U-Boot and ATF.
As a result, there maybe signal integrity on MMC connection due to
clock. So, the code should reports error to user when 'clksel' fail.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>