neigh: Remove check for AF_UNSPEC in rtnl_neigh_get()

This check was introduces to not accidently return AF_BRIDGE objects
to unaware API users as they do differ in structure. However, such
objects are only available if explicitely requests using the
NL_CACHE_AF_ITER flag or by using arg1 == AF_BRIDGE for the cache.

Therefore remove this check and allow rtnl_neigh_get() to be used to
fetch any neighbor object of a cache.

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Thomas Graf <tgraf@suug.ch>