Bluetoth: Host: Fix buffer allocation warnings in system workqueue

The buffer allocation in conn.c will trigger warnings if we try to use
anything else than K_NO_WAIT for the timeout when called from within the
system workqueue.

The calls in l2cap.c and att.c which may pass non-zero timeouts already
have proper handling for failed allocations, so make sure we use K_NO_WAIT
to avoid unnecessary warnings from conn.c.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>