usb: dwc3: msm: Rework dwc3_gadget_vbus_draw()

Since dwc3_gadget_vbus_draw() was merely calling
dwc3_otg_set_power() through usb_otg. Use notify_event() to notify
controller driver and call dwc3_msm_gadget_vbus_draw() which sets
the current to be drawn from USB.
This helps to remove the dependency on dwc3_otg from
dwc3_gadget.

Change-Id: Ie9000ddabedce667dd3251e9a0321c89fe12f602
Signed-off-by: Jack Pham <jackp@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>