wifi: mt76: fix airoha_npu dependency tracking

wifi: mt76: fix airoha_npu dependency tracking

There is a new build failure with MT7996E=m MT76_CORE=y and NET_AIROHA_NPU=m:

ld.lld: error: undefined symbol: airoha_npu_get
ld.lld: error: undefined symbol: airoha_npu_put
>>> referenced by npu.c
>>>               drivers/net/wireless/mediatek/mt76/npu.o:(mt76_npu_init) in archive vmlinux.a

Fix this by reworking the dependency for the MT7996_NPU to only
allow enabling that when mt76_core can link against the npu driver.

To make sure this gets caught more easily in the future when additional
mt76 variants need the same dependency, also turn CONFIG_MT76_NPU into
a tristate symbol that has the same dependency.

Fixes: 7fb554b1b623 ("wifi: mt76: Introduce the NPU generic layer")
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patch.msgid.link/20260612201519.4054683-1-arnd@kernel.org
Signed-off-by: Felix Fietkau <nbd@nbd.name>