Bluetooth: Add debugfs entry for setting vendor diagnostic mode

This adds a new debugfs entry for enabling and disabling the vendor
diagnostic mode. It is only exposed for drivers that provide the
set_diag driver callback and actually have an option for vendor
specific diagnostic information.

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