TTY: amiserial, use only one copy of async flags

Huh, why would one want to store two copies of them? Get rid of the
one from async_struct. That structure is going away as a whole soon.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>