Staging: comedi: kcomedilib: comedi_do_insn is now static

No one else calls this function, so mark it static.
Now we can strip out the unneeded functionality in here as well.

Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>