forcedeth: remove unnecessary structure member

Since both tx_ring and first_tx are the head of tx ring, it not
necessary to use two structure members to statically indicate
the head of tx ring. So first_tx is removed.

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>