ttyprintk: Add TTY port shutdown callback

ttyprintk: Add TTY port shutdown callback

By adding the TTY port shutdown callback, the final buffer flush has
been moved from tpk_close() to tpk_port_shutdown(). This enables buffer
flush upon close and hangup as well. The change also removes the
specific 'buf == NULL' handling in tpk_printk(), which became useless.

Signed-off-by: Samo Pogačnik <samo_pogacnik@t-2.net>
Link: https://lore.kernel.org/r/f236ea26520fbef176fefec2e06a43548f63fa9c.camel@t-2.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>