sdhci-pci: Use MRFLD as abbreviation of Merrifield

Everywhere else in the code MRFLD abbreviation is used for Intel Merrifield. Do
the same for sdhci-pci.

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>