staging: comedi: mpc624: tidy up the ai conversion speed code

Rename the CamelCase private data 'ulConvertionRate' and MPC624_SPEED_*
defines.

Create a macro to set the OSR bits that set the ai conversion speed and
use it for the various MPC624_SPEED_* defines. Remove the unnecessary
comment describing the speeds, this information is alread in the comedi
driver comment block.

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>