wifi: mt76: mt7915: remove VHT160 capability on MT7915?

[Thomas Walker]

Doing a little more reading and guessing that this is related to
https://forum.openwrt.org/t/802-11ax-worse-than-802-11ac-with-mt76-driver/126466/408?

Giving it (VHT80 and explicitly enabling the coloring/beam forming ax
bits) a try.  I'm not really one to be running iperf against every
config change, nor do I have more than 1 or 2 ax clients so maybe I
simply missed the potential perf impact?  I just noticed when the
config I'd been using for 6+ months failed in a way that didn't
provide much of a useful error message and required bisecting to hunt
down.

(And a likely inconsequential error in my initial report, the device
is an RB03, not RB01.)