hwmon: (smm665): Return error from i2c_smbus_read_word_swapped

There is no reason to replace one error code with another,
and returning -1 as error code is wrong anyway.

Smatch log:
drivers/hwmon/smm665.c:225 smm665_read_adc() info: why not propagate 'rv'
from i2c_smbus_read_word_swapped() instead of (-1)?

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>