mlxsw: core: Prevent duplication during QSFP module initialization

Verify during thermal initialization if QSFP module's entry is already
configured in order to prevent duplication.
Such scenario could happen in case two switch drivers (PCI and I2C
based) coexist and if after boot, splitting configuration is applied
for some ports and then I2C based driver is re-probed.
In such case after reboot same QSFP module, associated with split will
be discovered by I2C based driver few times, and it will cause a crash.

It could happen for example on system equipped with BMC (Baseboard
Management Controller), running I2C based driver, when the next steps
are performed:
- System boot
- Host side configures port spilt.
- BMC side is rebooted.

Fixes: 6a79507c

 ("mlxsw: core: Extend thermal module with per QSFP module thermal zones")
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>