i2c: Convert SCx200 driver from using raw PCI to platform device

The SCx200 ACB driver supports ISA hardware as well as PCI.  The PCI
hardware is CS5535/CS5536 based, and the device that it grabs is handled by
the cs5535-mfd driver. This converts the SCx200 driver to use a
platform_driver rather than the previous PCI hackery.

The driver used to manually track the iface list (via linked list); now it
only does this for ISA devices.  PCI ifaces are handled through standard
driver model lists.

It's unclear what happens in case of errors in the old ISA code; rather than
pretending the code actually cares, I've dropped the (implicit) ignorance
of return values and marked it with a comment.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>