power: supply: surface-charger: Fix type of integer variable

The ac->state field is __le32, not u32. So change the variable we're
temporarily storing it in to __le32 as well.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: e61ffb34 ("power: supply: Add AC driver for Surface Aggregator Module")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>