mac80211: Allow disabling SGI-20

This allows user-space (wpa_supplicant) to disable
short guard interval (SGI) for 20Mhz.  The SGI-40
disable option is already handled.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index 1b087ff..0c0d4ca 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -537,6 +537,7 @@
 
 	.cap_info = cpu_to_le16(IEEE80211_HT_CAP_SUP_WIDTH_20_40 |
 				IEEE80211_HT_CAP_MAX_AMSDU |
+				IEEE80211_HT_CAP_SGI_20 |
 				IEEE80211_HT_CAP_SGI_40),
 	.mcs = {
 		.rx_mask = { 0xff, 0xff, 0xff, 0xff, 0xff,