usb: phy-generic: fix nop xceiv probe

Commit bd27fa44e13830d2baa278d5702e766380659cb3 (usb: phy: generic:
Don't use regulator framework for RESET line) introduced regression: All
users of usb_nop_xceiv_register() will fail because there is no platform
data and the default reset GPIO is 0 which is a valid GPIO. Fix that.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>