TTY: simserial no longer needs serialP

Let's do a spin-off of serial_state structure with only needed
elements.

And remove serialP crap from includes.

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>