staging: comedi: adv_pci1710: remove 'has_counter' boardinfo

All the boards supported by this driver have a 8254 counter. Channels
1 and 2 are used to create the cascaded 32-bit analog input pacer.
Counter 0 is available for the user on all the boards except the PCI-1713.

Remove the 'has_counter' boardinfo and use the 'is_pci1713' boardinfo to
determine if the user counter subdevice needs to be allocated and
initialized.

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>