Prefer ENETDOWN to ENXIO when returning queuing errors due to a link down, interface down, etc, with if_cxgb's if_transmit routine.
Prefer ENETDOWN to ENXIO when returning queuing errors due to a link
down, interface down, etc, with if_cxgb's if_transmit routine.

MFC after:	3 days
Reviewed by:	kmacy

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