serial: sirf: Staticize local symbols

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

drivers/tty/serial/sirfsoc_uart.c:147:6: warning: symbol 'sirfsoc_uart_start_tx' was not declared. Should it be static?
drivers/tty/serial/sirfsoc_uart.c:636:5: warning: symbol 'sirfsoc_uart_probe' was not declared. Should it be static?

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