Administration: Correct and simplify the logic for `asc` and `desc` arguments...

Administration: Correct and simplify the logic for `asc` and `desc` arguments in `WP_List_Table::get_sortable_columns()`.

Setting the initial order didn't work as expected due to reversed logic.

Follow-up to [48151].

See #45089.
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git-svn-id: https://core.svn.wordpress.org/trunk@47934 1a063a9b-81f0-0310-95a4-ce76da25c4cd