Bluetooth: Move eir_get_length() function into hci_event.c

The eir_get_length() function is only used from hci_event.c and so
instead of having a public function move it to the location where
it is used.

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