Staging: comedi: add comedi_num_legacy_minors module parameter

As suggested the legacy device count is set to zero. A new module
parameter for comedi_fops allows setting the number of legacy devices:
comedi_num_legacy_minors. The default is zero.

From: Bernd Porr <BerndPorr@f2s.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Frank Mori Hess <fmhess@users.sourceforge.net>
Cc: David Schleef <ds@schleef.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>