How to Stop WordPress Asking for FTP details to Upgrade Plugins:
If you install/upgrade WordPress plugin from WordPress admin area, you will need to specify FTP details each time. It is not a problem if you have few plugins but if you have installed many plugins, it consumes lots of time installing/upgrading plugin from WordPress admin.
To avoid this, you can directly specify your FTP details in your wp-config.php file. You just need to add following codes in your wp-config.php file:
// If SSL connection is requireed add following line as well
Save your wp-config.php file and try to install/upgrade plugin from your WordPress admin area.