ieee80211: add IEEE80211_COUNTRY_STRING_LEN definition

and make use of it in wireless drivers

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/at76c50x-usb.h b/drivers/net/wireless/at76c50x-usb.h
index 4a37447..f14a654 100644
--- a/drivers/net/wireless/at76c50x-usb.h
+++ b/drivers/net/wireless/at76c50x-usb.h
@@ -290,7 +290,7 @@
 	u8 res;
 	u8 multi_domain_capability_implemented;
 	u8 multi_domain_capability_enabled;
-	u8 country_string[3];
+	u8 country_string[IEEE80211_COUNTRY_STRING_LEN];
 	u8 reserved[3];
 } __packed;