bnxt_en: Let bnxt_stamp_tx_skb() return error code

Change the function bnxt_stamp_tx_skb() to return 0 for suceess
or -EAGAIN if the timestamp is still pending in firmware.  The
calling PTP aux worker will reschedule based on the return code.

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>