USB: cxacru: potential underflow in cxacru_cm_get_array()

The value of "offd" comes off the instance->rcv_buf[] and we used it as
the offset into an array.  The problem is that we check the upper bound
but not for negative values.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/atm/cxacru.c

@@ -686,7 +686,8 @@ static int cxacru_cm_get_array(struct cxacru_data *instance, enum cxacru_cm_requ
 {
 	int ret, len;
 	__le32 *buf;
-	int offb, offd;
+	int offb;
+	unsigned int offd;
 	const int stride = CMD_PACKET_SIZE / (4 * 2) - 1;
 	int buflen =  ((size - 1) / stride + 1 + size * 2) * 4;