net: ena: reduce driver load time

This commit reduces the driver load time by using usec resolution
instead of msec when polling for hardware state change.

Also add back-off mechanism to handle cases where minimal sleep
time is not enough.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Sameeh Jubran <sameehj@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>