staging: comedi: addi_apci_3xxx: tidy up i_APCI3XXX_InsnReadAnalogInput()

The analog input subdevice in this driver is broken. It abuses the
comedi API and will not work as-is. Start cleaning it up so it can
be fixed.

Rename the CamelCase function and the local variables.

Refactor the function to remove the indents. Most of the indents are
left over from the previous patches.

Invert the early tests to provide a quick exit from the function.

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>