staging: comedi: s626: remove unused DMAHandle

The `DMAHandle` member of `struct bufferDMA` in "s626.h" is unused, so
remove it.

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