mailbox: ti-msgmgr: Change message count mask to be descriptor based

Change mask used to extract the message count to be descriptor based.

This is to support changes for count location for various SoC
solutions.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
1 file changed