arm64: kexec_file: return successfully even if kaslr-seed doesn't exist

In kexec_file_load, kaslr-seed property of the current dtb will be deleted
any way before setting a new value if possible. It doesn't matter whether
it exists in the current dtb.

So "ret" should be reset to 0 here.

Fixes: commit 884143f6 ("arm64: kexec_file: add kaslr support")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>