bnxt_en: Reset PTP tx_avail after possible firmware reset

It is possible that during error recovery and firmware reset,
there is a pending TX PTP packet waiting for the timestamp.
We need to reset this condition so that after recovery, the
tx_avail count for PTP is reset back to the initial value.
Otherwise, we may not accept any PTP TX timestamps after
recovery.

Fixes: 118612d5 ("bnxt_en: Add PTP clock APIs, ioctls, and ethtool methods")
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
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>