staging: comedi: addi_apci_3120: enable chanlist scanning if needed

The 'SCAN_ENA' bit in the mode register needs to be set if the chanlist
has more than 1 channel. Set the bit in apci3120_set_chanlist() if needed.
The callers write the mode register after setting any additional bits,

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>