build breakage from of i2c helper patch

[Jochen Friedrich]

Hi Jean,

> (blind shot) Maybe this means that some code that is in i2c-core should
> be moved to i2c-boardinfo instead? i2c-boardinfo is always built into
> the kernel even when i2c-core is modular.

unfortunately, i2c_new_device() tries to attach the module, so this function
really needs full i2c-core functionality.

I think the best way to fix this is to make the OF helpers def_tristate I2C, so
it will be compiled as module if i2c-core is compiled as module, as well.

I'll send an updated patch in a minute.

Thanks,
Jochen