hwmon: (pmbus/ltc2978) Update datasheet links

Links to datasheets are no longer valid. Provide links to product information
instead (which provides links to the datasheets and is hopefully more
persistent).
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>

Documentation/hwmon/ltc2978

@@ -5,11 +5,11 @@ Supported chips:
   * Linear Technology LTC2978
     Prefix: 'ltc2978'
     Addresses scanned: -
-    Datasheet: http://cds.linear.com/docs/Datasheet/2978fa.pdf
+    Datasheet: http://www.linear.com/product/ltc2978
   * Linear Technology LTC3880
     Prefix: 'ltc3880'
     Addresses scanned: -
-    Datasheet: http://cds.linear.com/docs/Datasheet/3880f.pdf
+    Datasheet: http://www.linear.com/product/ltc3880
 
 Author: Guenter Roeck <linux@roeck-us.net>