wifi: mt76: mt7921: fix non-PSC channel scan fail

Due to the scan command may only request legacy bands and PSC channel
in 6GHz band, we are unable to scan the APs on non-PSC channel in this
case. Enable WIPHY_FLAG_SPLIT_SCAN_6GHZ to support non-PSC channel
(obtained during scan on legacy bands) in 6GHz scan request.

Fixes: 50ac15a5

 ("mt76: mt7921: add 6GHz support")
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>