mlxsw: core: Add missing rollback in error path

Without this rollback, the thermal zone is still registered during the
error path, whereas its private data is freed upon the destruction of
the underlying bus device due to the use of devm_kzalloc(). This results
in use after free.

Fix this by calling mlxsw_thermal_fini() from the appropriate place in
the error path.

Fixes: a50c1e35 ("mlxsw: core: Implement thermal zone")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>