serial: 8250_fintek: Make fintek_8250_set_termios static

Fix sparse warning:

drivers/tty/serial/8250/8250_fintek.c:306:6: warning:
 symbol 'fintek_8250_set_termios' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>