serial: core, remove uart_update_termios

Now, uart_update_termios is empty, so it's time to remove it. We no
longer need a live tty in .dtr_rts. So this should prune all the bugs
where tty is zeroed in port->tty during tty_port_block_til_ready.

There is one thing to note. We don't set ASYNC_NORMAL_ACTIVE now. It's
because this is done already in tty_port_block_til_ready.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>