net/bnxt: fix xstats

net/bnxt: fix xstats

If the HWRM_PORT_QSTATS_EXT fails to initialize
fw_rx_port_stats_ext_size or fw_tx_port_stats_ext_size,
the driver can end up passing junk statistics to the application.

Instead of relying on the application to initialize the xstats
buffer before calling the xstats_get dev_op, memset xstats
with zeros to avoid returning or displaying incorrect statistics.

Also fixed the buffer starting offset.

Fixes: f55e12f33416 ("net/bnxt: support extended port counters")
Cc: stable@dpdk.org
Signed-off-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Lance Richardson <lance.richardson@broadcom.com>