staging: comedi: mpc624: there are only 4 analog inputs

According to the datasheet, this board has 4 differential analog
input channels not 8. Fix the subdevice init.

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>