tty: serial_core: add tty NULL check to uart_tx_stopped

Commit 761ed4a94582 ("tty: serial_core: convert uart_close to use
tty_port_close") created a case where a port used for a console does not
get shutdown on tty closing. Then a call to uart_tx_stopped() segfaults
because the tty is NULL. This could be fixed to restore old behavior,
but we also want to allow tty_ports to work without a tty attached. So
this change to allow a NULL tty_struct is needed either way.

Fixes: 761ed4a94582
("tty: serial_core: convert uart_close to use tty_port_close")
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: Ifbeeacc89d396b857332ce94cc2d77bdd6e12bb7
Git-repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Git-commit: a727b025f43d7952c0697562f5cecda9f42758aa
Signed-off-by: Lei wang <leiwan@codeaurora.org>