staging: comedi: addi_apci_1564: fix counter code in main driver source

The Rev 1.0 APCI-1564 boards do not have counters.

Fix the code in the main driver source so that the I/O accesses to the
counters do not happen if the devpriv->counters member is not initialized.

This does not fix the code in hwdrv_apci1564.c. That code violates the
comedi API and is currently broken.

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>