USB: cxacru: return an empty value for modulation if there is no connection

When there is no connection, return an empty string
instead of "0" for the connection modulation.
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/networking/cxacru.txt

@@ -61,6 +61,7 @@ several sysfs attribute files for retrieving device statistics:
 * mac_address
 
 * modulation
+	"" (when not connected)
 	"ANSI T1.413"
 	"ITU-T G.992.1 (G.DMT)"
 	"ITU-T G.992.2 (G.LITE)"

drivers/usb/atm/cxacru.c

@@ -267,12 +267,12 @@ static ssize_t cxacru_sysfs_showattr_LINE(u32 value, char *buf)
 static ssize_t cxacru_sysfs_showattr_MODU(u32 value, char *buf)
 {
 	static char *str[] = {
-			NULL,
+			"",
 			"ANSI T1.413",
 			"ITU-T G.992.1 (G.DMT)",
 			"ITU-T G.992.2 (G.LITE)"
 	};
-	if (unlikely(value >= ARRAY_SIZE(str) || str[value] == NULL))
+	if (unlikely(value >= ARRAY_SIZE(str)))
 		return snprintf(buf, PAGE_SIZE, "%u\n", value);
 	return snprintf(buf, PAGE_SIZE, "%s\n", str[value]);
 }