Bluetooth: Host: Add l2cap credit param checks

Adds a missing requirement from Core Spec V6.0 Vol 3.A chapters 10.1
and 10.2 to ignore L2CAP_FLOW_CONTROL_CREDIT_IND packets with the
credit value set to 0.

Matches existing credit-related functions by checking that the CID
is in the dynamic range (you can't add credits to fixed channels).

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>