staging: comedi: addi_apci_3120: rename private data 'b_ModeSelectRegister'

Rename this CamelCase member of the private data and tidy up the mask/set
of its bits.

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>