Various bugfixes from Stefano Garzarella:

1. oce_multiq_start(): make sure the buffer is consumed even on ENXIO
2. oce_multiq_transmit(): there is an extra call to drbr_enqueue()
  causing the mbuf to be enqueued twice when the NIC's queue is full,
  and potential panics
3. oce_multiq_transmit(): same problem fixed recently in ixgbe (r267187)
   and other drivers: if the mbuf is enqueued, the proper return value is 0

Submitted by:	Stefano Garzarella
MFC after:	3 days