staging: comedi: addi_apci_3120: don't use timer 2 to count scans

Timer 2 can be used to count the number of scans when the cmd->stop_src
is TRIG_COUNT. Unfortunately that timer can also be used by userspace and
this breaks the async command. Timer 2 also only works for counting the
number of scans when DMA is not used.

The number of scans is counted by the core using the private data membmer
'scans_done'. Use that to detect the End-Of-Scan for both DMA and none
DMA modes.

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>