i2c: designware-pci: Make bus number allocation robust

On some platforms, such as Intel Medfield, the I2C slave devices are enumerated
through SFI tables where bus numbering is expected to be defined in the OS.
Make the bus number allocation robust for such platforms.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>