bnxt_en: Add is_ts_pkt field to struct bnxt_sw_tx_bd

Remove the unused is_gso field and add the is_ts_pkt field to struct
bnxt_sw_tx_bd.  This field will mark the TX BD that has requested
HW TX timestamp.  The field needs to be cleared if the timestamp packet
is later aborted.  This field will be useful when processing the
new TX timestamp completion from the hardware in the next patches.

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