staging: comedi: addi_apci_3120: tidy up timer 2 programming in apci3120_ai_cmd()

Tidy up the programming 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>