Bluetooth: Refactor read default sys config for various types

Refactor read default system configuration function so that it's capable
of returning different types than u16

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