Bluetooth: Add eir_len parameter to mgmt_ev_device_found

This patch add a two byte eir_len parameter mgmt_ev_device_found. Since
it's unlikely that the data will in the short term be much bigger than
conventional EIR lengths just use a small stack based buffer for now to
avoid dynamic memory allocation & freeing.

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