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

this warning:

  drivers/net/cassini.c: In function ‘cas_rx_ringN’:
  drivers/net/cassini.c:2350: warning: ‘skb’ may be used uninitialized in this function

triggers because GCC does not recognize the (correct) error flow
between cas_rx_process_pkt() and 'skb'.

Annotate it.

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