MdeModulePkg RamDiskDxe: Fix incorrect RAM disk memory address calculation

MdeModulePkg RamDiskDxe: Fix incorrect RAM disk memory address calculation

EndingAddress is calculated incorrectly. Original code calculates it as
(StartingAddress + Size). Correct value should be (StartingAddress +
Size - 1.

Note:
Besides the changes made by Samer, Hao also fixed a similar issue in
RamDiskImpl.c

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Samer El-Haj-Mahmoud <elhaj@hpe.com>
Signed-off-by: Tapan Shah <tapandshah@hpe.com>
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hpe.com>