Bluetooth: Fix EIR data generation for mgmt_device_found

The mgmt_device_found function expects to receive only the significant
part of the EIR data so it needs to be removed before calling the
function. This patch adds a new eir_get_length() helper function to
calculate the length of the significant part.

Signed-off-by: Vishal Agarwal <vishal.agarwal@stericsson.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>