staging: comedi: gsc_hpdi: remove hpdi_writel()

This helper function is used to OR bits with a software copy of a
register value then writel() the new value to the register. The
software copies are never updated in the driver so they are always
0 due to the kzalloc.

Remove the unnecessary 'bits' from the private data and replace
the hpdi_writel() calls with writel().

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>