bnxt_en: Add TX timestamp completion logic

The new BCM5760X chips will return the timestamp of TX packets in a
new completion.  Add logic in __bnxt_poll_work() to handle this
completion type to retrieve the timestamp.  This feature eliminates
the limit on the number of in-flight PTP TX packets.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>