staging: comedi: rtd520: remove RtdDma0Next macro

This macro uses the 'devpriv' macro which relies on a local variable
having a specific name. Plus it's just a wrapper around a simple
'writel'. Remove the macro.

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