powerpc/prom_init: Fix kernel config grep
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.15.53 by commit 0c1d781d6b08b1b8942bb70421f494057fc6950c, it was introduced in version v5.2 by commit 26deb04342e343ac58ab05bc7d2345ff0be9b667. For more details please see the references link.