power: supply: core: fix clang -Wunsequenced

The increment operator of  pointer in be32_to_cpu() is not explicitly.
It made the warning from clang:

drivers/power/supply/power_supply_core.c:674:36: error: multiple
unsequenced modifications to 'list' [-Werror,-Wunsequenced]
drivers/power/supply/power_supply_core.c:675:41: error: multiple
unsequenced modifications to 'list' [-Werror,-Wunsequenced]

Link: https://github.com/ClangBuiltLinux/linux/issues/460



Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>