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

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>