hwmon: (max6621) Inverted if condition in max6621_read()

We intended to test for failure here but accidentally tested for
success.  It means that we don't set "*val" to true and it means that
if i2c_smbus_write_byte() does fail then we return success.

Fixes: e7895864b0d7 ("hwmon: (max6621) Add support for Maxim MAX6621 temperature sensor")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>