Bluetooth: Track LE L2CAP credits in l2cap_chan

This patch adds tracking of L2CAP connection oriented channel local and
remote credits to struct l2cap_chan and ensures that connect requests
and responses contain the right values.

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