bnxt_en: Refactor all PTP TX timestamp fields into a struct

On the older 5750X (P5) chips, we currently support only 1 TX PTP
packet in-flight waiting for the timestamp.  Refactor the
datastructures to prepare to support up to 4 TX PTP packets.

Combine all fields required for PTP TX timestamp query into one
structure.  An array of this structure will be added in follow-on
patches to support multiple outstanding TX timestamps.

Signed-off-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>