staging: comedi: addi_apci_3501: use addi_tcw.h for the timer registers

The APCI3501_TIMER_* register defines in this driver are that same as
the ADDI_TCW_* defines with an additional offset.

Add a 'tcw' (timer/counter/watchdog) member to the private data to
hold the address for the iobase of the registers. Use that and the
defines from addi_tcw.h to address the registers.

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>