Bluetooth: Rename L2CAP_CHAN_CONN_FIX_A2MP to L2CAP_CHAN_FIXED

There's no reason why A2MP should need or deserve its on channel type.
Instead we should be able to group all fixed CID users under a single
channel type and reuse as much code as possible for them. Where CID
specific exceptions are needed the chan-scid value can be used.

This patch renames the current A2MP channel type to a generic one and
thereby paves the way to allow converting ATT and SMP (and any future
fixed channel protocols) to use the new channel type.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>