mt76: mt7915: fix phy cap in mt7915_set_stream_he_txbf_caps()

This patch fixes performance issue of beamforming tx on
bandwidth 160MHz.

Fixes: 99ad32a4

 ("mt76: mt7915: add support for MT7986")
Signed-off-by: Peter Chiu <chui-hao.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>