BaseTools/DevicePath: use explicit 64-bit number parsing routines

BaseTools/DevicePath: use explicit 64-bit number parsing routines

Replace invocations of StrHexToUintn() with StrHexToUint64(), so
that we can drop the former.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>