Bluetooth: Unify mode related management messages to a single struct

The powered, connectable and discoverable messages all have the same
format. By using a single struct for all of them a lot of code can be
simplified and reused.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>