staging: comedi: hwdrv_apci3501: remove "magic" numbers in apci3501_read_insn_timer()

Use register bit defines from addi_tcw.h to remove the "magic" numbers.

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>