Bluetooth: mgmt: Fix clearing of hdev->eir

The hdev->eir buffer needs to be cleared when clearing the EIR data.
Otherwise subsequent attempts at setting the EIR to something valid
again may fail because the code thinks that the EIR hasn't changed.

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