usb: phy: generic: fix the vbus interrupt request

Declare the interrupt as "one shot" so that it is masked until the end
of the threaded handler. This prevents the irq core from spitting out an
error :
  "Threaded irq requested with handler=NULL and !ONESHOT for irq 63"

This was introduced by commit "usb: phy: generic: add vbus support".

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>