staging: comedi: ni_tiocmd: tidy up ni_tio_configure_dma()

The 'enable' and 'read_not_write' parameters are true/false flags. For
aesthetics, change their types to bool.

Rename the local variable 'input_select_bits' to simply 'bits' and reuse
it when enabling the dma on the m series and 660x counter variants.

Add a local variable for the 'mask' that is passed to ni_tio_set_bits()
to help clarify the code.

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>