powerpc/64s: Fix unnecessary copy to 0 when kernel is booted at address 0

According to the code logic, when the kernel is loaded at address 0, no
copying operation should be performed, but it is currently being done.

This patch fixes the issue where the kernel code was incorrectly
duplicated to address 0 when booting from address 0.

Fixes: b270bebd ("powerpc/64s: Run at the kernel virtual address earlier in boot")
Cc: stable@vger.kernel.org # v6.4+
Signed-off-by: Jinglin Wen <jinglin.wen@shingroup.cn>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240620024150.14857-1-jinglin.wen@shingroup.cn