Bluetooth: mgmt: Refactor hci_dev lookup for commands

Almost all mgmt commands need to lookup a struct hci_dev based on the
index received within the mgmt headers. It makese therefore sense to
look this up in a single place and then just pass the hdev pointer to
each command handler function.
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>
Acked-by: NMarcel Holtmann <marcel@holtmann.org>