uio: Statically allocate uio_class and use class .dev_attrs.

Instead of adding uio class attributes manually after the uio device has
been created and we have sent a uevent to userspace, use the class
attribute mechanism.  This removes races and makes the code simpler.

At the same time don't bother to dynamically allocate a struct class for
uio, just declare one statically.  Less code is needed and it is easier
to set the class parameters.tune the class

Signed-off-by: Eric W. Biederman <ebiederm@aristanetworks.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Hans J. Koch <hjk@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>