usb: dwc3: fix the glue drivers using the nop phy

The reset_gpio member of the usb_phy_gen_xceiv_platform_data
structure needs the have negative value or phy-generic's
probe will fail unless DT is used. 0 is a valid gpio number.

This fixes an issue where phy-generic fails to probe with
message: "usb_phy_gen_xceiv.0: Error requesting RESET GPIO 0".

Cc: <stable@vger.kernel.org>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>