net/sfc: maintain management event queue

net/sfc: maintain management event queue

The event queue is required for device level events (e.g. link status
change) and flush events.
Provide thread-safe function to poll the event queue since it may be
really done from different contexts.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@solarflare.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>