mlxsw: spectrum: Do not enforce same firmware version for multiple ASICs

In commit a72afb68 ("mlxsw: Enforce firmware version for
Spectrum-2") I added a required firmware version for Spectrum-2, but
missed the fact that mlxsw_sp2_init() is used by both Spectrum-2 and
Spectrum-3. This means that the same firmware version will be used for
both, which is wrong.

Fix this by creating a new init() callback for Spectrum-3.

Fixes: a72afb68 ("mlxsw: Enforce firmware version for Spectrum-2")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Tested-by: Shalom Toledo <shalomt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>