Bluetooth: Add support for max_tx_power in Get Conn Info

This patch adds support for max_tx_power in Get Connection Information
request. Value is read only once for given connection and then always
returned in response as parameter.

Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>