TTY: simserial, use only one copy of async flags

The same as for amiserial. Use only one instance of the flags.

Also remove them from async_struct now. Nobody else uses them.

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