OvmfPkg/AcpiPlatformDxe: drop double right shift in ADD/WRITE POINTER cmds

OvmfPkg/AcpiPlatformDxe: drop double right shift in ADD/WRITE POINTER cmds

The Count parameter of RShiftU64() must be strictly smaller than 64.
ProcessCmdAddPointer() and ProcessCmdWritePointer() currently ensure this
by "cleverly" breaking the last bit of a potentially 8-byte right shift
out to a separate operation.

Instead, exclude the Count==64 case explicitly (in which case the
preexistent outer RShiftU64() would return 0), and keep only the inner
RShiftU64(), with the direct Count however.

This is not a functional change, just style improvement.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>