net/dpaa: fix xstats implementation
net/dpaa: fix xstats implementation

Some of the applications are expecting that if n is 0, it shall
return the supported number of stats. e.g. VPP

Fixes: b21ed3e2a16d ("net/dpaa: support extended statistics")
Cc: stable@dpdk.org

Signed-off-by: Hemant Agrawal <90d96ef1ecd8ed49b97c7897be950e80824a9c5c@nxp.com>
Acked-by: Shreyansh Jain <9744844f062a9f6cf95f66f06508da54c8fc7929@nxp.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk