bnxt_en: Fix xmit_more with BQL.

We need to write the doorbell if BQL has stopped the queue and
skb->xmit_more is set.  Otherwise it is possible for the tx queue to
rot and cause tx timeout.

Fixes: 4d172f21 ("bnxt_en: Implement xmit_more.")
Suggested-by: Yuval Mintz <yuval.mintz@cavium.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>