ar71xx: tp-link.mk: always include device version in image and DEVICE_TITLE

There are currently several supported TP-Link devices without specified
version number in image name and/or DEVICE_TITLE (e.g. WBS210, WBS510,
TL-WR810N, TL-WA7510N, TL-WPA8630), but vendor website shows that there
are already more than one version of them on the market.

For devices like Archer C5, which second version is based on a total
different platform, missing version number in DEVICE_TITLE (used in
menuconfig) might be misleading for users.

To make it less confusing for users and easier to maintain in future,
include version number in image name and DEVICE_TITLE for all TP-Link
devices, even if there is only one version of device at the moment.

Also, keep DEVICE_TITLE in same format for all TP-Link devices.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
Acked-by: John Crispin <john@phrozen.org>