TTY: simserial, define local tty_port pointer

And use it to make the code more readable.

Since tport doesn't conflict with port anymore and there are not many
tport accessors left, do also s/\<tport\>/port/g.

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>