ArmPkg: fix compilation error in ArmDmaLib

ArmPkg: fix compilation error in ArmDmaLib

Commit 0a99a65d2c8a ("fix incorrect device address of double buffer")
retained an explicit cast on the variable "Buffer" which became
incorrect with the other changes, leading to compilation failures
with some toolchains. Drop the cast.

Contributed-under: TianoCore Contribution Agreement 1.0
Reported-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>