mlxsw: spectrum_router: Rename nexthop update function to reflect its type

mlxsw_sp_nexthop_update() is used to update the configuration of
Ethernet-type nexthops, as opposed to mlxsw_sp_nexthop_ipip_update(),
which is used to update IPinIP-type nexthops.

Rename the function to mlxsw_sp_nexthop_eth_update(), so that it is
consistent with mlxsw_sp_nexthop_ipip_update().

It will allow us to introduce mlxsw_sp_nexthop_update() in a follow-up
patch, which calls either of above mentioned function based on the
nexthop's type.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>