staging: comedi: cb_pcidas: cleanup ai_config_insn()

Absorb the helper function ai_config_calibration_source() into
ai_config_insn() and remove the static const variable that
was in the helper function.

Return an error code (-EINVAL) when appropriate and the number
of data values used (insn->n) for success.

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>