Bluetooth: Remove return values from functions that don't need them

There are many functions that never fail but still declare an integer
return value for no reason. This patch converts these functions to use a
void return value to avoid any confusion of whether they can fail or not.

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