Bluetooth: Reorder L2CAP functions to avoid forward declarations

This patch moves the l2cap_conn_add, is_valid_psm and l2cap_chan_connect
functions further down in l2cap_core.c. The patch doesn't contain
anything else except the relocation of these functions. By moving the
functions further down the patch enables a subsequent patch that adds a
pending RX queue to be implemented without a forward declaration of a
function.

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