common/cnxk: fix SQ flush sequence

common/cnxk: fix SQ flush sequence

Fix SQ flush sequence to issue NIX RX SW Sync after SMQ flush.
This sync ensures that all the packets that were in-flight are
flushed out of memory.

This patch also fixes NULL return issues reported by
static analysis tool in Traffic Manager and sync's mailbox
to that of the kernel version.

Fixes: 05d727e8b14a ("common/cnxk: support NIX traffic management")
Fixes: 0b7e667ee303 ("common/cnxk: enable packet marking")
Cc: stable@dpdk.org
Signed-off-by: Satha Rao <skoteshwar@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>