mlxsw: spectrum_router: Introduce FIB entry update op

Follow-up patchset introducing XMDR implementation is going to need
to distinguish write and update ops. Therefore introduce "update op"
and call "write op" only when new FIB entry is inserted.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>