Bluetooth: Pass command length to LE signaling channel handlers

The LE signaling PDU length is already calculated in the
l2cap_le_sig_channel function so we can just pass the value to the
various handler functions to avoid unnecessary recalculations (byte
order conversions). Right now the only user is the connection parameter
update procedure, but as new LE signaling operations become available
(for connection oriented channels) they will also be able to make use of
the value.

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