Driver core: warn when userspace writes to the uevent file in a non-supported way

In the future we will allow the uevent type to be written to the uevent
file to trigger the different types of uevents.  But for now, as we only
support the ADD event, warn if userspace tries to write anything else to
this file.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>