drivers: usb_dc_mcux_ehci: rework USB_DeviceNotificationTrigger()

Rework USB_DeviceNotificationTrigger(). Fix style.

Drop messages from USB_DeviceEhciCancel().
MCUX EHCI driver notifies about canceled transfers,
but there is no specific code for this event in
usb_device_callback_message_struct_t, the only way to
recognize it is to check the length value.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>