TTY: rfcomm/tty, use count from tty_port

This means converting an atomic counter to a counter protected by
lock. This is the first step needed to convert the rest of the code to
the tty_port helpers.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>