i2c: piix4: Move SMBus controller base address detect into function

Move SMBus controller base address detection into function. Refactor
is in preparation for following MMIO changes.

Signed-off-by: Terry Bowman <terry.bowman@amd.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>