usb: phy: ab8500: use correct enum type

The local variable event is of type enum usb_phy_events. Use
the same enum value USB_EVENT_NONE instead of UX500_MUSB_NONE.

This avoids a warning when building with clang:
  drivers/usb/phy/phy-ab8500-usb.c:906:30: warning: implicit conversion from
  enumeration type 'enum ux500_musb_vbus_id_status' to different enumeration
  type 'enum usb_phy_events' [-Wenum-conversion]
        enum usb_phy_events event = UX500_MUSB_NONE;
                            ~~~~~   ^~~~~~~~~~~~~~~

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>