USB: storage: sierra_ms: fix sysfs file attribute

A non-writable sysfs file shouldn't have writable attributes.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kevin Lloyd <klloyd@sierrawireless.com>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/storage/sierra_ms.c

@@ -121,7 +121,7 @@ static ssize_t show_truinst(struct device *dev, struct device_attribute *attr,
 	}
 	return result;
 }
-static DEVICE_ATTR(truinst, S_IWUGO | S_IRUGO, show_truinst, NULL);
+static DEVICE_ATTR(truinst, S_IRUGO, show_truinst, NULL);
 
 int sierra_ms_init(struct us_data *us)
 {