mailbox: bcm-flexrm-mailbox: Fix mask used in CMPL_START_ADDR_VALUE()

The mask used in CMPL_START_ADDR_VALUE() should be 27bits instead of
26bits. This incorrect mask was causing completion writes to 40bits
physical address fail.

This patch fixes mask used in CMPL_START_ADDR_VALUE() macro.

Fixes: dbc049ee ("mailbox: Add driver for Broadcom FlexRM
ring manager")

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