staging: comedi: s626: tidy up freeing of the dma buffers

Currently the dma buffers are freed during the detach of the driver using
the function s626_close_dma_b(). The buffers are also freed while the
interrupt handler is still attached. This could result in a race condition.

Refactor the code so that the buffers are freed after the interrupt handler
is released and the PCI device is disabled.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>