sanitize slash in model name for tp-link-archer-c6-v2-eu-ru-jp

[maurerle]

The Archer C6 v2 has the weird name: tp-link-archer-c6-v2-v2 (EU/RU/JP) - this is not a valid file name for pictures as it includes slashes.

For example here: https://firmware.freifunk-aachen.de/?q=TP-Link%E2%81%A3%20Archer%C2%A0C6%E2%81%A3%20v2%C2%A0(EU%2FRU%2FJP)%E2%81%A3 The proper pictures are called tp-link-archer-c6-v2.jpg or tp-link-archer-c6-v2-eu-ru-jp.jpg and are available.

The model name should be sanitized in this case to request the tp-link-archer-c6-v2-eu-ru-jp.jpg

The alternative is to change the entry in devices.js