staging: comedi: Simplify NI driver configuration

At the moment, the options to select drivers for various National
Instruments boards in the "Comedi PCI drivers", "Comedi ISA and PC/104
drivers" and "Comedi PCMCIA drivers" menus do not appear unless you
first enable and enter the "Comedi National Instruments card support"
menu and possibly select some of the options in there.  This is pretty
confusing for the user.

Get rid of the "Comedi National Instruments card support" menu
(CONFIG_COMEDI_NI_COMMON) and make the CONFIG_COMEDI_MITE,
CONFIG_COMEDI_NI_TIO and CONFIG_COMEDI_NI_TIOCMD options selectable by
other options instead of dependencies of those other options.

We lose the ability to build the 'mite', 'ni_tio' and 'ni_tiocmd'
modules independently of other modules, but they are not useful on their
own anyway.

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