nl80211: fix validation of mesh path nexthop

Mesh path nexthop should be a ethernet address, but current validation
checks against 4 byte integers.

Cc: stable@vger.kernel.org
Fixes: 2ec600d6

 ("nl80211/cfg80211: support for mesh, sta dumping")
Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20191029093003.10355-1-markus.theil@tu-ilmenau.de
Signed-off-by: Johannes Berg <johannes.berg@intel.com>