staging: comedi: das08: rename DAS08AO_AO_LSB() and DAS08AO_AO_MSB()

The `DAS08AO_AO_LSB(x)` macro returns the offset to the analog output
low byte register for channel x (0 or 1) for "AOL", "AOM", and "AOH"
boards.  The `DAS08AO_AO_MSB(x)` macro returns the offset to the
corresponding high byte register.  Rename the macros to
`DAS08AOX_AO_LSB_REG(x)` and `DAS08AOX_AO_MSB_REG(x)` respectively, and
add some comments.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>