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

Tidy up the programming of timers 0 and 1 so that only one local variable is
required.

Also, remove the unnecessary clear of devpriv->timer_mode. This value will get
set correctly by each apci3120_timer_set_mode() call. It's not necessary to
clear it first.

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>