mailbox: bcm-flexrm-mailbox: Fix FlexRM ring flush timeout issue

mainline inclusion
from mainline-v5.0-rc7
commit d7bf31a0
category: bugfix
bugzilla: 10755
CVE: NA

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

RING_CONTROL reg was not written due to wrong address, hence all
the subsequent ring flush was timing out.

Fixes: a371c10e ("mailbox: bcm-flexrm-mailbox: Fix FlexRM ring flush sequence")

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Signed-off-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>