Bluetooth: Host: Add req/rsp l2cap validation

L2CAP channels will now, along with the ident, store the opcode of the
pending request. This commit expands the ident lookup function to also
compare received response types to this opcode, and will ignore
unsolicited responses.

Setting of idents for channels are moved after verification of buffer
allocation for the request to be sent. A TODO is added for improving
this functionality at a later time.

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