rtnetlink: Drop unnecessary return value from ndo_dflt_fdb_del

This change cleans up ndo_dflt_fdb_del to drop the ENOTSUPP return value since
that isn't actually returned anywhere in the code.  As a result we are able to
drop a few lines by just defaulting this to -EINVAL.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>