staging: comedi: drivers: fix return value of comedi_load_firmware()

Some of the callback functions that upload the firmware in the comedi
drivers return a positive value indicating the number of bytes sent
to the device. Detect this condition and just return '0' to indicate
a successful upload.

Reported-by: Bernd Porr <mail@berndporr.me.uk>
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>