Now that noone checks the return value, switch the return type of
tty_unregister_driver to void. We can do that as we always return zero.

Generally, drivers are not allowed to call tty_unregister_driver while
there are open devices.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-35-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>