mmc: sdhci-cadence: refactor probe function

Use added dev variable for devm_clk_get.

Signed-off-by: Piotr Sroka <piotrs@cadence.com>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>