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

Determine the chan, range, and aref when the variables are declared.

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>