usbnet: silence an unnecessary warning

That a kevent could not be scheduled is not an error.
Such handlers must be able to deal with multiple events anyway.
As the successful scheduling of a work is a debug event, make
the failure debug priority, too.

V2: coding style

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by: Cristian Caravena <caravena@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>