staging: comedi: cb_pcidas: absorb cal_enable_bits()

This inline function just returns the bits needd to enable
a calibration source. For aethetics, absorb it into the callers.
Sorten the variable name in the private data used to hold the
current calibration source.

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>