staging: comedi: ni_labpc: cleanup local var declarations in labpc_ai_cmd()

Determine the scan_mode early and get the proper chanspec so that
the chan, range, and aref can be determined when the variables are
declared.

labpc_ai_chanlist_invalid() checks that all the range and aref values
in the chanlist are the same so, for consistency, use the same chanspec
used to get the chan.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>