arm: pmu: Improve PMU error reporting

The KVM ioctls mostly just return -1 in the error case, leaving the
actual error code in errno.

Change the output of the PMU error message to actually print this error
code instead of the generic -1.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>