Bluetooth: Simplify l2cap_chan initialization for LE CoC

The values in l2cap_chan that are used for actually transmitting data
only need to be initialized right after we've received an L2CAP Connect
Request or just before we send one. The only thing that we need to
initialize though bind() and connect() is the chan->mode value. This way
all other initializations can be done in the l2cap_le_flowctl_init
function (which now becomes private to l2cap_core.c) and the
l2cap_le_flowctl_start function can be completely removed.

Also, since the l2cap_sock_init function initializes the imtu and omtu
to adequate values these do not need to be part of l2cap_le_flowctl_init.

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