wifi: qtnfmac: use struct_size and size_sub for payload length

Replace the calculations for the payload length in
qtnf_cmd_band_fill_iftype with struct_size() and size_sub(). While
the payload length does not get directly passed to an allocation function,
the performed calculation is still calculating the size of a flexible array
structure (minus the size of a header structure).
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Igor Mitsyanko <imitsyanko@quantenna.com>
Cc: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230307230212.3735818-1-jacob.e.keller@intel.com

drivers/net/wireless/quantenna/qtnfmac/commands.c

@@ -1325,9 +1325,10 @@ static int qtnf_cmd_band_fill_iftype(const u8 *data,
 	struct ieee80211_sband_iftype_data *iftype_data;
 	const struct qlink_tlv_iftype_data *tlv =
 		(const struct qlink_tlv_iftype_data *)data;
-	size_t payload_len = tlv->n_iftype_data * sizeof(*tlv->iftype_data) +
-		sizeof(*tlv) -
-		sizeof(struct qlink_tlv_hdr);
+	size_t payload_len;
+
+	payload_len = struct_size(tlv, iftype_data, tlv->n_iftype_data);
+	payload_len = size_sub(payload_len, sizeof(struct qlink_tlv_hdr));
 
 	if (tlv->hdr.len != cpu_to_le16(payload_len)) {
 		pr_err("bad IFTYPE_DATA TLV len %u\n", tlv->hdr.len);