staging: comedi: me_daq: tidy up ai fifo/chanlist register defines

Tidy up this register usage by defining some macros to set the channel
and gain bits. Add a define for the bit that enables differential mode.

Writing to this offset puts data in the ai chanlist fifo, reading from
it gets data from the ai data fifo. For aesthetics, use the same define
to read and write the registers.

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>