mailbox: Hi3660: Fixup mailbox state machine malfunction issue

Current mailbox driver of Hi3660 release the mailbox directly
before sending a new message which may cause last message lost
and next message sending doesn't take effect actually.

This patch fixs this issue by following the right progress below,
each time before sending a message, mailbox driver will check
whether the mailbox is in ready state, if last message has been
acknowledged, the mailbox driver will clear the ack state to turn
the mailbox to ready state again.

Signed-off-by: Kevin Wangtao <kevin.wangtao@hisilicon.com>
Reviewed-and-tested-by: Leo Yan <leo.yan@linaro.org>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>