bnxt_en: Remove an impossible condition check for PTP TX pending SKB

In the current 5750X PTP code paths, there is always at most one TX
SKB requested for timestamp and we won't accept another one until we
have retrieved the timestamp or it has timed out.  Remove the
unnecessary check in bnxt_get_tx_ts_p5() for a pending SKB and change
the function to void.

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>