pch_gbe: simplify transmit time stamping flag test

This patch makes logic surrounding the test of the
transmit time stamping flag more readable.

[ RC - Rebased Takahiro's changes and wrote a commit message
  explaining the changes. ]
Signed-off-by: Takahiro Shimizu <tshimizu818@gmail.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/oki-semi/pch_gbe/pch_gbe_main.c

@@ -199,11 +199,11 @@ static void pch_tx_timestamp(
 	u32 cnt, val;
 
 	shtx = skb_shinfo(skb);
-	if (unlikely(shtx->tx_flags & SKBTX_HW_TSTAMP && adapter->hwts_tx_en))
-		shtx->tx_flags |= SKBTX_IN_PROGRESS;
-	else
+	if (likely(!(shtx->tx_flags & SKBTX_HW_TSTAMP && adapter->hwts_tx_en)))
 		return;
 
+	shtx->tx_flags |= SKBTX_IN_PROGRESS;
+
 	/* Get ieee1588's dev information */
 	pdev = adapter->ptp_pdev;