backlight: ipaq_micro: Fix sparse non static symbol warning

Fixes the following sparse warnings:

drivers/video/backlight/ipaq_micro_bl.c:73:24: warning:
 symbol 'micro_backlight_device_driver' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>