vtgpu: Fix the physical memory size

Use the correct endian switching function when switching to a little
endian 64-bit address. Even on a little-endian machine this will
truncate the address to a 32-bit value.

Approved by: re (gjb)
Sponsored by: Arm Ltd

(cherry picked from commit 4386935191c576fa62a52d52734e264fe8329a67)
(cherry picked from commit 5df4deb384d3af12b10bc6de0788578d6a56e207)