[U-Boot] Regression: dm: i2c: Make i2c_get_chip_for_busnum() fail if the chip is not detected

[Stephen Warren]

The following commit:

> dm: i2c: Make i2c_get_chip_for_busnum() fail if the chip is not detected
>     
>     i2c_get_chip_for_busnum() really should check the presence of the chip on
>     the bus. Most of the users of this function assume that this is done.

... causes a boot failure on NVIDIA Jetson TX2:

> U-Boot 2019.01-rc1-00220-g7ff485c68b7e (Dec 10 2018 - 11:20:41 -0700)
> 
> TEGRA186
> Model: NVIDIA P2771-0000-500
> DRAM:  7.8 GiB
> tegra_ivc_read_get_next_frame() timed out (-12)
> tegra_board_init: Cannot find MAX77620 I2C chip
> initcall sequence 00000000fffa95a8 failed at call 0000000080083480 (err=-110)
> ### ERROR ### Please RESET the board ###

This may be due to the fact the bus in question is implemented by RPC to 
a separate CPU, and that mechanism hasn't been used with probing before. 
In general though, there's not guarantee that probing will work even on 
a local/native I2C bus, since different chips don't support all probe 
methods (see i2c-detect in Linux, which supports various different 
probing methods due to this), so I'm rather surprised this change was 
implemented. Is it really necessary? I believe we should revert it.

Thanks.