staging: comedi: adl_pci9118: convert private data true/false flags to bit-fields

For aesthetics convert the private data true/false flags to bit-fields. The
'usemux' member is used in the driver as a flag. Refactor the analog input
subdevice init so this member can also be a bit-field.

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>