staging: comedi: pcmda12: remove 'BITS' define

'BITS' is a pretty generic name for a define. It's only used in
the board attach to set the subdevice 'maxdata' so instead of
renaming the define just open code the value and remove the define.

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>