usb: dwc3: gadget: Refactor pullup()

Move soft-disconnect sequence out of dwc3_gadget_pullup(). No
functional change here.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/4c0f259b17d95acaaa931f90276683a48a32fe22.1650593829.git.Thinh.Nguyen@synopsys.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>