power: fix environment detection

Anything coming from sysfs has a newline at the end. Cut it off before
comparing the strings.

Fixes: 20ab67608a39 ("power: add environment capability probing")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: David Hunt <david.hunt@intel.com>
Tested-by: Lihong Ma <lihongx.ma@intel.com>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>