MIPS: Fix potential build failures using cpu_vpe_id on non-MT

MIPS: Fix potential build failures using cpu_vpe_id on non-MT

When used in a non-MT kernel, the cpu_vpe_id macro never made use of
its cpuinfo argument. It doesn't actually need to since it is returning
a constant 0. However not using the argument can lead to build failures
if the compiler then notices that a variable used as part of the
argument is unused. Prevent that problem by "using" the argument as far
as the compiler is concerned, whilst still returning 0 as before.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7280/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>