Bluetooth: Only one command per L2CAP LE signalling is supported

The Bluetooth specification makes it clear that only one command
should be present in the L2CAP LE signalling packet. So tighten
the checks here and restrict it to exactly one command.

This is different from L2CAP BR/EDR signalling where multiple
commands can be part of the same packet.

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