usb: dwc3: Fix probe deferral issue

regulator_get will return -EPROBE_DEFER if named supply is present
in devicetree, but the regulator driver has not probed.

Change-Id: Ieef7c133e92a35d881fd6fcebd88df38a1b60357
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>