s2io: fix warning in drivers/net/s2io.c

this warning:

  drivers/net/s2io.c: In function ‘rx_intr_handler’:
  drivers/net/s2io.c:7369: warning: ‘lro’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between:

 - s2io_club_tcp_session()and 'lro'

Annotate it.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>