staging: comedi: c6xdigio: introduce c6xdigio_write_data()

All writes to the hardware involve writing the the data register then
checking the status register. Introduce a helper function to handle this.

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>