mt76: mt7915: potential array overflow in mt7915_mcu_tx_rate_report()

Smatch complains that "wcidx" value comes from the network and thus
cannot be trusted.  In this case, it actually seems to come from the
firmware.  If your wireless firmware is malicious then probably no
amount of carefulness can protect you.

On the other hand, these days we still try to check the firmware as much
as possible.  Verifying that the index is within bounds will silence a
static checker warning.  And it's harmless and a good exercise in kernel
hardening.  So I suggest that we do add a bounds check.

Fixes: e57b7901

 ("mt76: add mac80211 driver for MT7915 PCIe-based chipsets")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>