The greth_start_xmit_gbit() returns NETDEV_TX_OK without freeing skb
in case of skb->len being too long, add dev_kfree_skb() to fix it.

Fixes: d4c41139 ("net: Add Aeroflex Gaisler 10/100/1G Ethernet MAC driver")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Link: https://patch.msgid.link/20241012110434.49265-1-wanghai38@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>