net: enetc: use the skb variable directly in enetc_clean_tx_ring()

The code checks whether the skb had one-step TX timestamping enabled, in
order to schedule the work item for emptying the priv->tx_skbs queue.

That code checks for "tx_swbd->skb" directly, when we already had a skb
retrieved using enetc_tx_swbd_get_skb(tx_swbd) - a TX software BD can
also hold an XDP_TX packet or an XDP frame. But since the direct tx_swbd
dereference is in an "if" block guarded by the non-NULL quality of
"skb", accessing "tx_swbd->skb" directly is not wrong, just confusing.

Just use the local variable named "skb".

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>