Bluetooth: Add hdev->short_name for EIR generation

It's possible to provide a short name through the mgmt interface and
this name can be used for EIR generation when the full name doesn't fit
there. This patch adds the preliminary tracking of the provided short
name.

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