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

Factor the freeing of the dma buffers out of the (*detach).

Move the freeing of the buffers so that it occurs after the PCI device
has been disabled to avoid any race condition.

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>