net: Check for presence of IFLA_AF_SPEC

ndo_bridge_setlink() is currently only called on the slave if
IFLA_AF_SPEC is set but this is a very fragile assumption and may
change in the future.

Cc: Ajit Khaparde <ajit.khaparde@emulex.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>