Making sure struct bt_l2cap_chan has absolutely no members related
to dynamic channels.
That way we ensure that there is no overhead for a build where only
fixed channels are used.
It's not enough that the dynamic channel-related members are put behind
ifdefs - they should be completely moved out from the struct definition.
Furthermore, the public l2cap.h header file already has a struct
that's meant to be used for dynamic channels: struct bt_l2cap_le_chan!
However, currently dynamic channel support is a mess - it's a mix
between these two structs. The bt_l2cap_le_chan struct should really
be an extension of the bt_l2cap_chan struct, i.e. the former should
contain as a member the latter.
Signed-off-by: Lingao Meng <menglingao@xiaomi.com>