staging: comedi: register sysfs device attributes with driver core

Currently the sysfs device attributes are created by the comedi
core after each comedi device is created. This can lead to a race
condition where userspace gets an add event before the files are
created.

Register the device attributes with the comedi class so that the
driver core handles creating them and we avoid the race.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Cc: Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>