Update class-add-admin-favicon.php

Extended version of the custom favicon filter.

Example filter function for plugin or theme (this is for a CobaltApps Dynamik theme):

```php
add_filter( 'multisite_enhancements_favicon_path', 'fb_favicon_path', 10, 3 );
function fb_favicon_path($default, $id, $path_type) {
	$path = '';
	switch ($path_type) {
		case 'dir':
			$path = $_SERVER['DOCUMENT_ROOT'].'/';
		break;
		case 'url':
		default:
			$path = network_home_url();
		break;
	}
	
	if (get_blog_option($id, 'stylesheet') == 'dynamik-gen') {
		return $path.'wp-content/uploads/sites/'.$id.'/dynamik-gen/theme/images/favicon.png';
	} else {
		return $default;
	}
}
```