ena: Introduce shared sample interval for all stats

Rename sample_interval node to stats_sample_interval and move
it up in the sysctl tree to make it clear that it's relevant for
all the stats and not only ENI metrics (Currently, sample interval node
is found under eni_metrics node).

Path to node:
dev.ena.<device_index>.stats_sample_interval

Once this parameter is set it will set the sample interval for all the
stats node including SRD/customer metrics.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.