bnxt_en: fix atomic counter for ptp packets

atomic_dec_if_positive returns new value regardless if it is updated or
not. The commit in fixes changed the behavior of the condition to one
that differs from original code. Restore original condition to properly
maintain atomic counter.

Fixes: 165f8769 ("bnxt_en: add timestamping statistics support")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240604091939.785535-1-vadfed@meta.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -513,7 +513,7 @@ static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		struct bnxt_ptp_cfg *ptp = bp->ptp_cfg;
 
 		if (ptp && ptp->tx_tstamp_en && !skb_is_gso(skb)) {
-			if (!atomic_dec_if_positive(&ptp->tx_avail)) {
+			if (atomic_dec_if_positive(&ptp->tx_avail) < 0) {
 				atomic64_inc(&ptp->stats.ts_err);
 				goto tx_no_ts;
 			}