power: ipaq-micro-battery: freeing the wrong variable

We accidentally free "micro_ac_power" which is an error pointer and it
leads to an oops.  We intended to free "micro_batt_power".

Fixes: a2c1d531

 ('power_supply: ipaq_micro_battery: Check return values in probe')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>