serial: timbuart: Staticize local symbols

These local symbols are used only in this file.
Fix the following sparse warnings:

drivers/tty/serial/timbuart.c:165:6: warning: symbol 'timbuart_handle_rx_port' was not declared. Should it be static?
drivers/tty/serial/timbuart.c:187:6: warning: symbol 'timbuart_tasklet' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>