sdhci-pci: refactor intel_mrfld_mmc_probe_slot()

Refactor intel_mrfld_mmc_probe_slot() to use switch case. The change allows to
add a support for SD and SDIO interfaces without any pain.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>