TTY: simserial, stop using serial_state->{line,icount}

* instead of line, use tty->index or an iterator
* icount is not made public, only the tx path increments it

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>