Bluetooth: Add HCI Read Data Block Size function

Implement block size read function. Use different variables for
packet-based and block-based flow control.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>