powerpc/prom_init: Fix shellcheck warnings

Fix "Double quote to prevent globbing and word splitting."
warning from shellcheck

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Stephen Rothwell <sfr@cab.auug.org.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250518044107.39928-3-maddy@linux.ibm.com