wifi: mt76: enable page_pool stats

Enable page_pool ethtool statistics for mt7915 and mt7921 chipsets.

Tested-by: Felix Fietkau <nbd@nbd.name>
Tested-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>