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 <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>