staging: comedi: adv_pci1710: remove 'n_aichan' boardinfo

This member of the boardinfo isn't really necessary. All the boards
have analog inputs, the pci1713 has 32 channels the rest have 16
channels.

There is already a 'is_pci1713' member in the boardinfo so that can
be used to determine the number of channels for the analog input
subdevice.

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>