Bluetooth: Add timeout field to mgmt_set_discoverable

Based on the revised mgmt API set_discoverable has a timeout parameter
to specify how long the adapter will remain discoverable. A value of 0
means "indefinitively".

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