Increase buf size in cmd_sta_set_wireless_vht()

The previously used buf size is not sufficient to handle recently added
new command like MCS_FixedRate, so increase buf size from 30 to 60.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
diff --git a/sta.c b/sta.c
index 7ded7ed..dba7783 100644
--- a/sta.c
+++ b/sta.c
@@ -6419,7 +6419,7 @@
 	const char *intf = get_param(cmd, "Interface");
 	const char *val;
 	const char *program;
-	char buf[30];
+	char buf[60];
 	int tkip = -1;
 	int wep = -1;