ramips: mt7621-dts: fix compatible string for mt7530 on TP-Link EAP615-Wall

ramips: mt7621-dts: fix compatible string for mt7530 on TP-Link EAP615-Wall

The correct compatible string for the multi-chip module MT7530 switch in
MT7621AT, MT7621DAT and MT7621ST SoCs is mediatek,mt7621. Remove the
property on TP-Link EAP615-Wall which will default to the said string.

Fixes: a1b8a4d7b3ff ("ramips: support TP-Link EAP615-Wall")
Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/net/dsa/mediatek,mt7530.yaml
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>