hwmon: (atxp1) Set and use error code from vid_to_reg()

vid_to_reg() returns -1 if it encounters an error. Return -EINVAL instead.
Its only caller, atxp1_storevcore(), doesn't use the return code but
returns -1 instead, which is wrong anyway as it means -EPERM.
Use the return value from vid_to_reg() instead to report the error.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <khali@linux-fr.org>