mmc: renesas_sdhi: Use devm_clk_get_optional() to obtain CD clock

Use the existing devm_clk_get_optional() helper to obtain the optional
Card Detect clock, instead of open-coding the same operation.
a side effect, real errors will now be handled correctly instead of
being ignored.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/540d803d31bf9aa1d0f78f431cae0ccd05387edc.1637069733.git.geert+renesas@glider.be