power: qpnp-smbcharger: add support to skip USB notifications

Some hardware configurations allow the USB driver to detect presence
and type without charger's APSD. This is true especially when the external
muxes are used to multiplex an USB port and a docking station on the usbin
input line.

The USB port when active, ends up notifying the USB driver via a gpio.
When docking station is active, USB side need not know about it.

Note that this change also removes the null checks for usb_psy. It is
initialized at probe and probe's success is dependent on proper usb_psy
detection and initialization.

Change-Id: I7e300ce19d82861f34ccf37545f3317221f727a9
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>