bnxt_en: Increase the max total outstanding PTP TX packets to 4

Start accepting up to 4 TX TS requests on BCM5750X (P5) chips.
These PTP TX packets will be queued in the ptp->txts_req[] array
waiting for the TX timestamp to complete.  The entries in the
array will be managed by a producer and consumer index.  The
producer index is updated under spinlock since multiple TX rings
can try to send PTP packets at the same time.

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>