usb: dwc3-msm: Defer probe early if vbus_regulator get fails

DWC3-msm driver is responsible for adding dwc3 core platform
device using the DWC3 core DT node passed as its child.
Since, there is no clean way to undo the effects of
of_platform_populate API, move regulator_get for Vbus early in
probe so that dwc3-msm probe can be deferred way before adding
its children platform devices i.e. dwc3 core and XHCI.

Change-Id: Id33b6b4e30e31468603c2984538d8a1c2da2a2ba
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>