Bluetooth: Move bt_accept_enqueue() to outside __l2cap_chan_add

bt_accept_enqueue() is not really a channel action, so do it outside.
This patch is part of a set of patches to create an struct l2cap_chan to
have a clear separation between the struct sock and the L2CAP channel
stuff.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>