mailbox: sprd: correct definition of SPRD_OUTBOX_FIFO_FULL

stable inclusion
from stable-5.10.20
commit 37aba9cfece7819127e5e0d739ff54ec44695b9b
bugzilla: 50608

--------------------------------

[ Upstream commit 4450f128 ]

According to the specification, bit[2] represents SPRD_OUTBOX_FIFO_FULL,
not bit[0], so correct it.

Fixes: ca27fc26 ("mailbox: sprd: Add Spreadtrum mailbox driver")
Signed-off-by: Magnum Shan <magnum.shan@unisoc.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>