Drop GREAT_SNR definition from 30 to 25 dB

This allows throughput estimates and 5 GHz preference over 2.4 GHz band
to be used in more cases. The previously used value of 30 was
significantly more conservative than the SNR limits used for the highest
rate in scan_est_throughput() and this resulted in cases where 5 GHz AP
was ignored while SNR with it would have been close to reaching the
maximum TX rate.

Signed-off-by: Jouni Malinen <j@w1.fi>

wpa_supplicant/scan.c

@@ -1788,10 +1788,12 @@ struct wpabuf * wpa_scan_get_vendor_ie_multi(const struct wpa_scan_res *res,
  * This doc https://supportforums.cisco.com/docs/DOC-12954 says, "the general
  * rule of thumb is that any SNR above 20 is good." This one
  * http://www.cisco.com/en/US/tech/tk722/tk809/technologies_q_and_a_item09186a00805e9a96.shtml#qa23
- * recommends 25 as a minimum SNR for 54 Mbps data rate. 30 is chosen here as a
- * conservative value.
+ * recommends 25 as a minimum SNR for 54 Mbps data rate. The estimates used in
+ * scan_est_throughput() allow even smaller SNR values for the maximum rates
+ * (21 for 54 Mbps, 22 for VHT80 MCS9, 24 for HT40 and HT20 MCS7). Use 25 as a
+ * somewhat conservative value here.
  */
-#define GREAT_SNR 30
+#define GREAT_SNR 25
 
 #define IS_5GHZ(n) (n > 4000)