serial: omap: fix potential NULL pointer dereference in serial_omap_runtime_suspend()

The dereference to 'up' should be moved below the NULL test.
Introduced by commit ddd85e22


(serial: omap: prevent runtime PM for "no_console_suspend")

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>