net/ena: fix getting xstats global stats offset

net/ena: fix getting xstats global stats offset

There was a bug in a code, which was reading stat_offset value from the
ena_stats_rx_strings array instead of ena_stats_global_strings.

It wasn't causing real problems just because ena_stats_rx_strings was
not smaller than ena_stats_global_strings and both arrays hold the same
offsets.

Fixes: 7830e905b7c9 ("net/ena: expose extended stats")
Cc: stable@dpdk.org
Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>