net: dm9051: Fix missing dev_kfree_skb() in dm9051_loop_rx()

The dm9051_loop_rx() returns without release skb when dm9051_stop_mrcmd()
returns error, free the skb to avoid this leak.

Fixes: 2dc95a4d ("net: Add dm9051 driver")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>