ipaq_micro_battery: fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/power/ipaq_micro_battery.c:278:24: warning:
 symbol 'micro_batt_device_driver' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NSebastian Reichel <sre@kernel.org>

drivers/power/ipaq_micro_battery.c

@@ -275,7 +275,7 @@ static const struct dev_pm_ops micro_batt_dev_pm_ops = {
 	SET_SYSTEM_SLEEP_PM_OPS(micro_batt_suspend, micro_batt_resume)
 };
 
-struct platform_driver micro_batt_device_driver = {
+static struct platform_driver micro_batt_device_driver = {
 	.driver		= {
 		.name	= "ipaq-micro-battery",
 		.pm	= &micro_batt_dev_pm_ops,