sis900: Fix mem leak in sis900_rx error path

Fix memory leak in error path of sis900_rx(). If we don't do this we'll
leak the skb we dev_alloc_skb()'ed just a few lines above when the
variable goes out of scope.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>