ath79: correctly define WiFi switch for TL-WR841ND v8

ath79: correctly define WiFi switch for TL-WR841ND v8

The TL-WR841ND v8 feature a WiFi switch instead of a button.
This adds the corresponding input-type to prevent booting into
failsafe regularly.

This has been defined correctly in ar71xx, but was overlooked
when migrating to ath79. In contrast, the TL-WR842ND v2, which
has the key set up as switch in ar71xx, actually has a button.
The TL-MR3420 v2 has a button as well and is set up correctly
for both targets. (Information based on TP-Link user guide)

Note:

While looking into this, I found that support PR for TL-MR3420 v2
switched reset button to ACTIVE_HIGH. However, the other two
device still use ACTIVE_LOW. This seems strange, but I cannot
verify it lacking the affected devices.

Fixes: FS#2733
Fixes: 9601d94138de ("add support for TP-Link TL-WR841N/ND v8")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>