i2c-mpc: suppress I2C device probing

This patch suppresses I2C device probing by clearing the class field
of the "struct i2c_adapter" for the MPC I2C bus adapters. Some board
configurations which rely on probing must be fixed up by adding a
proper I2C device node to the DTS file, like the TQM85xx modules.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>