hwmon: (max6639) Introduce local dev variable, and reduce noisiness

Introduce local variable 'dev' in probe function to make code easier
to read. Also drop dev_info message in probe to reduce noisiness
during boot.

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