tty_wakeup is safe to be called from all contexts. No need to schedule
a tasklet for that. Let's call it directly like in other drivers.

This allows us to kill another member of async_struct structure. (If
we remove the dummy uses in simserial.)
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>