Staging: comedi: kcomedilib: simplify comedi_do_insn()

Now that we know we are only making 2 different types of instructions,
only handle those two types.

Also make the call a bit more typesafe by passing the correct pointer
type.

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