mailbox: sprd: correct definition of SPRD_OUTBOX_FIFO_FULL

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>