staging: greybus: uart: fix tty use after free

commit 92dc0b1f upstream.

User space can hold a tty open indefinitely and tty drivers must not
release the underlying structures until the last user is gone.

Switch to using the tty-port reference counter to manage the life time
of the greybus tty state to avoid use after free after a disconnect.

Fixes: a18e1517

 ("greybus: more uart work")
Cc: stable@vger.kernel.org      # 4.9
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210906124538.22358-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>