The C standard is vague about the signedness of enums, but in this case
here, they are treated as unsigned so the error handling does not work.
Use an int type to fix this.

Fixes: c6ed48ef ("power: supply: add ChromeOS EC based charge control driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/ZoWKEs4mCqeLyTOB@stanley.mountainSigned-off-by: Tzung-Bi Shih <tzungbi@kernel.org>