octeontx2-cpt: Wake up waiting process as last step

commit 14c5933361262482a2a100bd7d0546628688b69d from
git@git.assembla.com:cavium/WindRiver.linux.git

The moment msgs_acked is updated then the process waiting after
sending a mailbox message wakes up instantly. Currently this causes
mailbox errors since new message created will be zeroed with
otx2_mbox_reset_msgs. Fix it by updating msgs_acked as the last
step in the otx2_cptpf_afpf_mbox_handler.

Change-Id: I9583869c14034f55a387d7f3c4635d8387b960c2
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/121055
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>