staging: comedi: addi_apci_1564: board has 3 timers

According to ADDI-DATA, the APCI-1564 only has 3 timers. There are some
customer specific boards with 4 timers but they use out of tree drivers.

Now that the timer 'channels' are handled correctly in the subdevice
functions, fix the number of timer channels for the subdevice and remove
the code that would access a 4th timer.

Also, remove the unnecessary initialzation of the subdevice 'len_chanlist'.
That member is only used by subdevices that support async commands.

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>