TTY: bfin_jtag_comm, use tty from tty_port

Switch from mutex to tty_port->lock and to tty refcounting. This needs
a 'continue' to be added to re-grab a tty after schedule returns.

And since tty is not protected by bfin_jc_tty_mutex remove it as well.
But this needs tty_port->count to be protected by tty_port->lock now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: uclinux-dist-devel@blackfin.uclinux.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>