uio: Don't clear driver data

Currently uio sets it's driver data to NULL just as it is unregistering
attributes.  sysfs maks the guaranatee that it will not call attributes
after device_destroy is called so this is unncessary and leads to lots
of unnecessary code in uio.c

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>