net: ravb: Do not apply features to hardware if the interface is down

Do not apply features to hardware if the interface is down. In case runtime
PM is enabled, and while the interface is down, the IP will be in reset
mode (as for some platforms disabling the clocks will switch the IP to
reset mode, which will lead to losing register contents) and applying
settings in reset mode is not an option. Instead, cache the features and
apply them in ravb_open() through ravb_emac_init().

To avoid accessing the hardware while the interface is down
pm_runtime_active() check was introduced. Along with it the device runtime
PM usage counter has been incremented to avoid disabling the device clocks
while the check is in progress (if any).

Commit prepares for the addition of runtime PM.

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>