i2c: piix4: Fix SMBus port selection for AMD Family 17h chips

AMD Family 17h uses the KERNCZ SMBus controller. While its documentation
is not publicly available, it is documented in the BIOS and Kernel
Developer’s Guide for AMD Family 15h Models 60h-6Fh Processors.

On this SMBus controller, the port select register is at PMx register
0x02, bit 4:3 (PMx00 register bit 20:19).

Without this patch, the 4 SMBus channels on AMD Family 17h chips are
mirrored and report the same chips on all channels.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org