staging: comedi: s626: fix the number of dio channels

The first digital i/o subdevice has its n_chan set to S626_DIO_CHANNELS
which is defined as 48. This is actually the total number of channels
provided by all three digital i/o subdevices. Each subdevice only has
16 channels.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>