Check that port is started when MAC filter is set

The MAC filter set may be called without softc_lock held in the case of
SIOCADDMULTI and SIOCDELMULTI ioctls. The ioctl handler checks IFF_DRV_RUNNING
flag which implies port started, but it is not guaranteed to remain.
softc_lock shared lock can't be held in the case of these ioctls processing,
since it results in failure where kernel complains that non-sleepable
lock is held in sleeping thread.

Both problems are repeatable on LAG with LACP proto bring up.

Submitted by: Andrew Rybchenko <Andrew.Rybchenko at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.

MFC after: 2 weeks