caif: unlock on error path in cfserl_receive()

There was an spin_unlock missing on the error path.  The spin_lock was
tucked in with the declarations so it was hard to spot.  I added a new
line.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Sjur Brændeland <sjurbren@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>