mt76: mt7921: fix build regression

mt76: mt7921: fix build regression

After mt7921s got added, there are two possible build problems:

a) mt7921s does not get built at all if mt7921e is not also enabled

b) there is a link error when mt7921e is a loadable module, but mt7921s
configured as built-in:

ERROR: modpost: "mt7921_mac_sta_add" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mac_sta_assoc" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mac_sta_remove" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mac_write_txwi" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mcu_drv_pmctrl" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mcu_fill_message" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_mcu_parse_response" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_ops" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_queue_rx_skb" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!
ERROR: modpost: "mt7921_update_channel" [drivers/net/wireless/mediatek/mt76/mt7921/mt7921e.ko] undefined!

Fix both by making sure that Kbuild enters the subdirectory when
either one is enabled.

Fixes: 48fab5bbef40 ("mt76: mt7921: introduce mt7921s support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20211204173848.873293-3-arnd@kernel.org