s390/3270: fix missing device_destroy() call

Unloading the fs3270 kernel module does not remove the created
"3270/tub" device. Reloading the module then causes a sysfs warning:
"sysfs: cannot create duplicate filename '/devices/virtual/3270/3270!tub'".

Call device_destroy() in the module exit function to solve this issue.

Signed-off-by: Hendrik Brueckner <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

drivers/s390/char/fs3270.c

···
 fs3270_exit(void)
 {
 	raw3270_unregister_notifier(&fs3270_notifier);
+	device_destroy(class3270, MKDEV(IBM_FS3270_MAJOR, 0));
 	__unregister_chrdev(IBM_FS3270_MAJOR, 0, 1, "fs3270");
 }