net: ena: add jiffies of last napi call to stats

There are instances when we want to know when the last napi was
called for debugging.

On stuck / heavy loaded CPUs, the ena napi handler might not be
called for a long period of time. This stat can help us to
determine how much time passed since the last execution of napi.

Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: Shay Agroskin <shayagr@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>