staging: comedi: addi_apci_3120: define the timer 2 operation bits

For aesthetics, redefine the bits in the mode register used to set the
operation mode of timer 2.

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>