MdeModulePkg/SdMmcPciHcDxe: Fix SdMmcMmcLegacy bus timing handling

MdeModulePkg/SdMmcPciHcDxe: Fix SdMmcMmcLegacy bus timing handling

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2218

There is a bug in current driver code that makes
it impossible for it to work with devices that
only work in legacy speed mode since the function
EmmcSwitchToHighSpeed will return with EFI_INVALID_PARAMETER
for such bus mode. Since the logic in that function will
work well for SdMmcMmcLegacy this patch just allows
to call this function with SdMmcMmcLegacy bus mode.

Signed-off-by: Mateusz Albecki <mateusz.albecki@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Tested-by: Hao A Wu <hao.a.wu@intel.com>
Tested-by: Marcin Wojtas <mw@semihalf.com>