pm2301-charger: Fix suspend/resume

The pm2301-charger driver implements runtime pm and at the same time uses
the legacy pm callbacks for suspend and resume. This does not work since
the I2C core wont look at the legacy pm callbacks if a driver has the 'pm'
field set. This patch fixes it by moving over to dev_pm_ops for
suspend/resume as well.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>