caif: Fix freezes when running CAIF loopback device

Fix spinlock bugs when running out of link-ids in loopback tests and
avoid allocating link-id when error is set in link-setup-response.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>