tty: remove tty NULL check from __do_SAK()

Both do_SAK_work() and vc_SAK() provide a valid tty to __do_SAK(), so
remove the check.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118071911.12059-3-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>