TTY: serialP, merge serial_state and async_struct

This is the final step to get rid of the one of the structures.  A
further cleanup will follow. And I struct serial_state deserves cease
to exist after a switch to tty_port too.

While changing the lines, it removes also pointless tty->driver_data
casts.

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>