net/bnxt: support fast mbuf free

Add support for DEV_TX_OFFLOAD_MBUF_FAST_FREE to bnxt
vector mode transmit. This offload may be enabled
only when multi-segment transmit is not needed, all
transmitted mbufs for a given queue will be allocated
from the same pool, and all transmitted mbufs will
have a reference count of 1.

Signed-off-by: Lance Richardson <lance.richardson@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>