Bluetooth: Move index to common header in management interface

Most mgmt commands and event are related to hci adapter. Moving index to
common header allow to easily use it in command status while reporting errors.
For those not related to adapter use MGMT_INDEX_NONE (0xFFFF) as index.

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>