staging: comedi: usbduxfast: document strange ai maxdata

The subdevice 'maxdata' is typically a mask of the valid bits that can
be returned by a subdevice, (1 << bits) - 1. The analog inputs of this
device have a resolution of 12-bits so the 'maxdata' should be 0x0fff.
But, this hardware can produce a value of 0x1000 indicating an overflow
from the ADC. The comedilib library's comedi_to_phys() function will
then return NAN when this value is read from the hardware.

Add a comment to clarify the strage 'maxdata' value.

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>