powerpc/kexec: Fix return of uninitialized variable

of_property_read_u64() can fail and leave the variable uninitialized,
which will then be used. Return error if reading the property failed.

Fixes: 2e6bd221 ("powerpc/kexec_file: Enable early kernel OPAL calls")
Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20240930075628.125138-1-zhangzekun11@huawei.com