s390/3270: fix use after free of tty3270_screen structure

The deactivation and freeing of the tty view of the 3270 device
can race with a tty3270_update invocation via the update timer.
To fix this move the del_timer_sync call for the update timer from
tty3270_free_view to tty3270_free prior to the tty3270_free_screen
call.

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