Bluetooth: centralize default value initialization.

This patch centralized the initialization of default parameters.  This
is required to allow clients to more easily customize the default
system parameters.

Signed-off-by: Alain Michaud <alainm@chromium.org>
Reviewed-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>