acpi-power-meter: Don't leak ACPI error codes to userspace

If the ACPI methods return an error code, we must return -EINVAL to userspace
to flag the error.  Right now we pass the (positive) number right through,
which causes echo to keep writing bogus values.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>