Bluetooth: Remove useless eir_len variable from EIR creation

The amount of data encoded so far in the create_eir() function can be
calculated simply through the difference between the data and ptr
pointer variables. The eir_len variable then becomes essentially
useless.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>