net: ravb: Return cached statistics if the interface is down

Return the cached statistics in case the interface is down. There should be
no drawback to this, as cached statistics are updated in ravb_close().

In order to avoid accessing the IP registers while the IP is runtime
suspended pm_runtime_active() check was introduced. The device runtime
PM usage counter has been incremented to avoid disabling the device clocks
while the check is in progress (if any).

The commit prepares the code for the addition of runtime PM support.

Suggested-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>