wifi: mt76: mt7915: validate skb length in txpower SKU query

wifi: mt76: mt7915: validate skb length in txpower SKU query

In mt7915_mcu_get_txpower_sku(), the response skb from
mt76_mcu_send_and_get_msg() is used in memcpy without validating
its length:

For TX_POWER_INFO_RATE:
  memcpy(res, skb->data + 4, sizeof(res));

where sizeof(res) is MT7915_SKU_RATE_NUM * 2 = 322 bytes.

For TX_POWER_INFO_PATH:
  memcpy(txpower, skb->data + 4, len);

In both cases, if the firmware returns a response shorter than
the expected size, the memcpy reads beyond the skb data buffer.
The data surfaces to userspace via debugfs (txpower_sku and
txpower_path).

Add length checks for both code paths before the memcpy.

Signed-off-by: Aviel Zohar <avielzohar123@gmail.com>
Link: https://patch.msgid.link/20260413033136.5417-3-avielzohar123@gmail.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>