make sure ifp->if_transmit returns 0 if a buffer is enqueued.

make sure ifp->if_transmit returns 0 if a buffer is enqueued.
A similar fix should be applied to vmxnet, ixgbe, igb, i40e.
(some of them previously reported by Michael Tuexen)

Drivers using if_transmit are correct, and so are most of the
other drivers that reassing if_transmit.

Among other things, this bug causes panics when using netmap emulation
on top of generic drivers.

Approved by:	bryanv
MFC after:	3 days