ath79: fix support-list for TP-Link WDR3600

ath79: fix support-list for TP-Link WDR3600

The TP-Link WDR3600 shares the same machine-code in the ar71xx target,
thus expecting "tl-wdr4300" not "tl-wdr3600" in the support-list
metadata to allow non-forced sysupgrades from ar71xx to ath79.

With this, it is possible to flash a WDR4300 image on the WDR3600. It
is no problem however, as the only difference is the 5GHz WiFi chip
which has 3SS instead of 2SS. Both work with either image.

Signed-off-by: David Bauer <mail@david-bauer.net>