mt76: mt7915: make read-only array ppet16_ppet8_ru3_ru0 static const

Don't populate the read-only array ppet16_ppet8_ru3_ru0 on the stack but
instead make it static const. Also makes the object code a little smaller.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>