isp1704_charger: Detect charger after probe

If the device is booted up with cable connected, or the
module is loaded after plugging in the cable, the
notification has come and gone, so not relying on it at
probe time. Instead this checks the VBUS level manually
after probe.

Signed-off-by: Heikki Krogerus <heikki.krogerus@nokia.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>