dsp56k: prevent a harmless underflow

There is a mistake here where we don't allow "len" to be zero but we
allow negative lengths.  It's basically harmless in this case, but the
underflow makes my static checker complain.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/dsp56k.c

@@ -325,7 +325,7 @@ static long dsp56k_ioctl(struct file *file, unsigned int cmd,
 			if(get_user(bin, &binary->bin) < 0)
 				return -EFAULT;
 		
-			if (len == 0) {
+			if (len <= 0) {
 				return -EINVAL;      /* nothing to upload?!? */
 			}
 			if (len > DSP56K_MAX_BINARY_LENGTH) {