i2c: i801: Fix using mux_pdev before it's set

i801_probe_optional_slaves() is called before i801_add_mux().
This results in mux_pdev being checked before it's set by
i801_add_mux(). Fix this by changing the order of the calls.
I consider this safe as I see no dependencies.

Fixes: 80e56b86

 ("i2c: i801: Simplify class-based client device instantiation")
Cc: stable@vger.kernel.org
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>