mailbox: bcm-flexrm-mailbox: Fix FlexRM ring flush sequence

mailbox: bcm-flexrm-mailbox: Fix FlexRM ring flush sequence

As-per suggestion from FlexRM HW folks, we have to first set
FlexRM ring flush state and then clear it for FlexRM ring flush
to work properly.

Currently, the FlexRM driver has incomplete FlexRM ring flush
sequence which causes repeated insmod+rmmod of mailbox client
drivers to fail.

This patch fixes FlexRM ring flush sequence in flexrm_shutdown()
as described above.

Fixes: dbc049eee730 ("mailbox: Add driver for Broadcom FlexRM
ring manager")

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>