wan: cosa: drop chan->wsem on error path

The other paths all drop chan->wsem.  This was found by a static
checker (smatch).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jan "Yenya" Kasprzak <kas@fi.muni.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>