staging: comedi: rti800: tidy up rti800_do_insn_bits()

To clarify the function a bit, add local variables for the 'mask'
and 'bits', passed by the comedi core, used to update the digital
outputs.

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>