bnxt_en: Remove atomic operations on ptp->tx_avail

bnxt_en: Remove atomic operations on ptp->tx_avail

Now that we require the spinlock to protect ptp->txts_prod, change
ptp->tx_avail to non-atomic and protect it under the same spinlock.
Add a new helper function bnxt_ptp_get_txts_prod() to decrement
ptp->tx_avail under spinlock and return the producer.

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>