power: supply: rk817: Change rk817_chg_cur_to_reg to int

Change return value of rk817_chg_cur_to_reg from u8 to int. If the
function fails to find a suitable value it returns a -EINVAL, but
defined as a u8 it would not return correctly. Additionally, change
defined variable that stores return value from u8 to int.

Fixes: 11cb8da0 ("power: supply: Add charger driver for Rockchip RK817")
Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>