viridian: don't blindly write to 32-bit registers if 'mode' is invalid

viridian: don't blindly write to 32-bit registers if 'mode' is invalid

If hvm_guest_x86_mode() returns something other than 8 or 4 then
viridian_hypercall() will return immediately but, on the way out, will write
back status as if 'mode' was 4. This patch simply makes it leave the registers
alone.

NOTE: The formatting of the 'out' label and the switch statement are also
      adjusted as per CODING_STYLE.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>