mmc: sdhci-of-at91: fix MMC_DDR_52 timing selection

The controller has different timings for MMC_TIMING_UHS_DDR50 and
MMC_TIMING_MMC_DDR52. Configuring the controller with SDHCI_CTRL_UHS_DDR50,
when MMC_TIMING_MMC_DDR52 timings are requested, is not correct and can
lead to unexpected behavior.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Fixes: bb5f8ea4 ("mmc: sdhci-of-at91: introduce driver for the Atmel SDMMC")
Cc: <stable@vger.kernel.org> # 4.4+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>