tty/serial: serial8250_set_divisor() can be static

drivers/tty/serial/8250/8250_core.c:2503:6: sparse: symbol 'serial8250_set_divisor' was not declared. Should it be static?

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>