bridge: vni: Fix vni filter help strings

bridge: vni: Fix vni filter help strings

Add the missing 'vni' subcommand to the top level `bridge help`.
For `bridge vni { add | del } ...`, 'dev' is a mandatory argument.
For `bridge vni show`, 'dev' is an optional argument.

Fixes: 45cd32f9f7d5 ("bridge: vxlan device vnifilter support")
Reviewed-by: Petr Machata <petrm@nvidia.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>