TTY: amiserial/simserial, use tty_port

Add tty_port to serial_state and start using common tty port members
from tty_port in amiserial and simserial. The rest will follow one by
one.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>