Calling i2c set speed twice for i2c_mux_bus

[Michal Simek]

Hi Heiko and Simon,

I have find out one bug in i2c class. I am using zcu104 revC board which
has dts in mainline where i2c1 has i2c mux with some channels.
In DT clock-frequency = <400000>; is specified and it is read in
i2c_post_probe(). But i2c_mux_bus_drv is also UCLASS_I2C which means
that post probe is called for it too. And because clock-frequency
property is not there default 100k is used.

I think that is bug and should be fixed.
Heiko: Are you aware about this issue?

Simon: I think that make sense to use some flags to skip calling
post_probe in this case. Or any suggestion how to fix it would be great.

Thanks,
Michal