[PATCH] skge: get rid of warning on race

Get rid of warning in case of race with ring full and lockless
tx on the skge driver. It is possible to be in the transmit
routine with no available slots and already stopped.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>