usb: dwc2: gadget: disable phy before turning off power regulators

This patch fixes probe function to match the pattern used elsewhere in
the driver, where power regulators are turned off as the last element in
the device shutdown procedure.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>