mmc: sdhci-pxav2: add error handling of clk_prepare_enable()

There is no check if clk_prepare_enable() succeed in sdhci_pxav2_probe().

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>