USB: cxacru: fix an bounds check warning

This is a privileged operation so it doesn't matter much.  We use "tmp"
as an offset into an array.  If it were invalid we could read out of
bounds and trigger an oops if the memory is not mapped.  Plus it makes
static checkers complain.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/cxacru.c

@@ -476,6 +476,8 @@ static ssize_t cxacru_sysfs_store_adsl_config(struct device *dev,
 			return -EINVAL;
 		if (index < 0 || index > 0x7f)
 			return -EINVAL;
+		if (tmp < 0 || tmp > len - pos)
+			return -EINVAL;
 		pos += tmp;
 
 		/* skip trailing newline */