power_supply: Replace strict_strtol() with kstrtol()

The usage of strict_strtol() is not preferred, because strict_strtol() is
obsolete. Thus, kstrtol() should be used.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>