usb: device_next: rename Kconfig template instances count

The name and wording used in template can be misleading.
Use more accurate word instance instead of device since
an implementation does not necessarily have to implement
a device (in the sense of Zephyr device model), and if it
does then devicetree should be used to describe number of
instances as in the case of CDC ACM implementation.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>