usb: phy: generic: fix the gpios to be optional

All the gpios, ie. reset-gpios and vbus-detect-gpio, should be optional
and not prevent the driver from working. Fix the regression in the
behavior introduced by commit "usb: phy: generic: migrate to gpio_desc".

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>