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: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>