TTY: amiserial no longer needs serialP

amiserial is the last user of serialP.h. Let's move struct
serial_state directly to amiserial and remove serialP crap from
includes. Finally, remove the header from the tree completely.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>