power_supply: Replace strict_strtoul() with kstrtoul()

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is obsolete.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

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>