power: smb135x-charger: use fastchg for updating battery psy

The current code assumes that by the time a charger is plugged in
it will start charging. However, the charger may be configured to
have a long hold-off time, where it deliberately waits few milli-
seconds before charging. In such a case when power supply changed
is called when USB is inserted charging will not have started
and the userspace will not report charging.

Use the fastchg interrupt which is triggered when charging actually
begins to notify start of charging to the userspace.

Change-Id: I89ad7cc22a56a032d607309aa0f474bc043ed97d
CRs-Fixed: 620850
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>