omap2/3/4: serial: kill dev_attr_sleep_timeout sparse warn

Remove the following sparse warnings by declaring attr as static:
arch/arm/mach-omap2/serial.c:627:1: warning: symbol 'dev_attr_sleep_timeout'
was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/serial.c

@@ -558,7 +558,8 @@ static ssize_t sleep_timeout_store(struct device *dev,
 	return n;
 }
 
-DEVICE_ATTR(sleep_timeout, 0644, sleep_timeout_show, sleep_timeout_store);
+static DEVICE_ATTR(sleep_timeout, 0644, sleep_timeout_show,
+		sleep_timeout_store);
 #define DEV_CREATE_FILE(dev, attr) WARN_ON(device_create_file(dev, attr))
 #else
 static inline void omap_uart_idle_init(struct omap_uart_state *uart) {}