wifi: mt76: mt7996: fix max nss value when getting rx chainmask

wifi: mt76: mt7996: fix max nss value when getting rx chainmask

[ Upstream commit 361b59b6be7c33c43b619d5cada394efc0f3b398 ]

Since wiphy->available_antennas_tx now accumulates the chainmask of all
the radios of a wiphy, use phy->orig_antenna_mask to get the original
max nss for comparison.

Fixes: 69d54ce7491d ("wifi: mt76: mt7996: switch to single multi-radio wiphy")
Signed-off-by: StanleyYP Wang <StanleyYP.Wang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Link: https://patch.msgid.link/20251106064203.1000505-1-shayne.chen@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Sasha Levin <sashal@kernel.org>