staging: comedi: das08: rename DAS08_MUX() and DAS08_MUX_MASK

The `DAS08_MUX_MASK` macro is a bitmask for the control register
corresponding to the analog input multiplexor channel selection bits.
Rename it to `DAS08_CONTROL_MUX_MASK` and add a comment.  Note that the
current setting of the multiplexor can also be read from the same bit
positions in the status register, but the driver does not use it.  Add a
comment to that effect.

The `DAS08_MUX(x)` macro takes an analog input channel number and
returns the corresponding analog input multiplexor channel selection
bits for the control register.  Rename it to `DAS08_CONTROL_MUX(x)` and
add a comment.

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>