mlxsw: Invoke driver's init/fini methods only if defined

We are going to add a minimal driver on top of the mlxsw core
infrastructure, which will be mainly used for hardware monitoring in
Baseboard management controller (BMC) installations.

Unlike the switch drivers (e.g., spectrum, switchx2), this driver does not
initialize the ASIC and therefore doesn't need to implement the init() and
fini() methods in its 'mlxsw_driver' struct.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>