forcedeth: optimize the rx with likely

In the rx fastpath, the function netdev_alloc_skb rarely fails.
Therefore, a likely() optimization is added to this error check
conditional.

CC: Srinivas Eeda <srinivas.eeda@oracle.com>
CC: Joe Jin <joe.jin@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>