mmc: Fix bug to enable DDR on MMC

MMC_CAP_UHS_DDR50 is an SD card feature and not
MMC feature. We don't have to check for this flag
to enable DDR on MMC. Checking for MMC_CAP_1.8V_DDR
is sufficient.

Change-Id: I6f44cb1f4dde6fe70cc816a2119c7b2b90944500
Signed-off-by: Viswanath Puttagunta <vishp@ti.com>