hw/net/virtio-net:Remove redundant statement in virtio_net_rsc_tcp_ctrl_check()

hw/net/virtio-net:Remove redundant statement in virtio_net_rsc_tcp_ctrl_check()

Clang static code analyzer show warning:
hw/net/virtio-net.c:2077:5: warning: Value stored to 'tcp_flag' is never read
    tcp_flag &= VIRTIO_NET_TCP_FLAG;
    ^           ~~~~~~~~~~~~~~~~~~~

The 'VIRTIO_NET_TCP_FLAG' is '0x3F'. The last ‘tcp_flag’ assignment statement is
 the same as that of the first two statements.

Reported-by: Euler Robot <[email protected]>
Signed-off-by: Chen Qun <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Li Qiang <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Message-Id: <20200827110311[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>