net/sfc/base: add API to get currently operating filters

net/sfc/base: add API to get currently operating filters

Unknown unicast filter creation may fail because of insufficient
permissions on VF. This failure is handled internally in libefx MAC
reconfiguration without any way for a user to know if it happened.
Making the MAC reconfiguration forward error code of filter
reconfiguration would be too destructive to the existing code
that may rely on the function never returning that error.

Add an API for getting the status of current unknown unicast and
all multicast filters since user must know that requested
filters are actually applied.

Signed-off-by: Igor Romanov <igor.romanov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>