staging: comedi: comedi_isadma: fix "stalled" detect in comedi_isadma_disable_on_sample()

The "stalled" variable this function is used to detect if the DMA operation
is stalled while trying to disable DMA on a full comedi sample. The reset
of this variable should only occur when the remaining bytes of the DMA
transfer does not equal the remaining bytes from the last check.

Reported-by: coverity (CID 1271132)
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>