usb: dwc3: Fix remove function if there is no ulpi_reset gpio

As ulpi_reset gpio is now optional, we need to check it when doing
the 'dwc3_generic_remove' function. Check if it is declared before
accessing the ulpi_reset.

Fixes: 237d1f60 ("usb: dwc3: Use the devm_gpiod_get_optional()
		     API for reset gpio")

Reported-by: Thomas Nizan <tnizan@witekio.com>
Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>