powerpc/prom_init: Fix improper check of prom_getprop()

prom_getprop() can return PROM_ERROR. Binary operator can not identify
it.

Fixes: 94d2dde7 ("[POWERPC] Efika: prune fixups and make them more carefull")
Signed-off-by: Peiwei Hu <jlu.hpw@foxmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/tencent_BA28CC6897B7C95A92EB8C580B5D18589105@qq.com