staging: comedi: ni_daq_700: add AI subdevice

Add subdevice 1 as an analog input (AI) subdevice.  It currently only
supports basic, software-triggered acquisitions.

This is mostly the work of Fred Brooks (MODULE_AUTHOR), but he based his
update on an older version of the driver.  I applied the relevant
changes with a few tweaks: adding an explicit `udelay(1)` in a timeout
loop, replacing binary constants with hex, renaming functions, replacing
`printk()` calls, removing exported symbols, removing (very) incomplete
comedi "command" support, and making some coding-style changes.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>