arm64/kernel: Fix return value when cpu_online() fails in __cpu_up()

If boot_secondary() was successful, and cpu_online() was an error in
__cpu_up(), -EIO was returned, but 0 is returned by commit d22b115c
("arm64/kernel: Simplify __cpu_up() by bailing out early").
Therefore, bringup_wait_for_ap() causes the primary core to wait for a
long time, which may cause boot failure.
This commit sets -EIO to return code under the same conditions.

Fixes: d22b115c ("arm64/kernel: Simplify __cpu_up() by bailing out early")
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
Tested-by: Yuji Ishikawa <yuji2.ishikawa@toshiba.co.jp>
Acked-by: Will Deacon <will@kernel.org>
Cc: Gavin Shan <gshan@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20200527233457.2531118-1-nobuhiro1.iwamatsu@toshiba.co.jp
[catalin.marinas@arm.com: return -EIO at the end of the function]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/smp.c

@@ -176,7 +176,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle)
 		panic("CPU%u detected unsupported configuration\n", cpu);
 	}
 
-	return ret;
+	return -EIO;
 }
 
 static void init_gic_priority_masking(void)