staging: vt6656: clean function's error path in usbpipe.c

Avoid discarding called function's returned value. Store it instead in
order to act accordingly.

Update error path to return 0 on success and a negative errno value on
error.

Signed-off-by: Quentin Deslandes <quentin.deslandes@itdev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>