The greth_init_rings() function won't free the newly allocated skb when
dma_mapping_error() returns error, so add dev_kfree_skb() to fix it.

Compile tested only.

Fixes: d4c41139 ("net: Add Aeroflex Gaisler 10/100/1G Ethernet MAC driver")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/1670134149-29516-1-git-send-email-zhangchangzhong@huawei.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>