docs: hwmon: (pmbus/ltc2978) Update datasheet URLs to analog.com.

Signed-off-by: Mike Jones <michael-a1.jones@analog.com>
Link: https://lore.kernel.org/r/1580234400-2829-1-git-send-email-michael-a1.jones@analog.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>