mlxsw: core: Rename cooling device

Rename cooling device from "Fan" to "mlxsw_fan".  Name "Fan" is too
common name, and such name is misleading, while it's interpreted by
user. For example name "Fan" could be used by ACPI.

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