selftests/powerpc/pmu: Fix unsigned function returning negative constant

The function __perf_reg_mask has an unsigned return type, but returns a
negative constant to indicate an error condition. So we change unsigned
to int.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1650788802-14402-1-git-send-email-baihaowen@meizu.com