i2c: piix4: Move SMBus port selection into function

i2c: piix4: Move SMBus port selection into function

commit fbafbd51bff52cb3a920fd98d4dae2a78dd433d0 upstream.

Move port selection code into a separate 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>
Cc: Mario Limonciello <Mario.Limonciello@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>