i2c: designware: Default to fast mode in case of ACPI

There is no way in ACPI to tell in which speed the host controller is
supposed to run, so we default to fast mode (400KHz). Since this has been
the default all the time there should be no functional changes with this
change.

This is the first step required to refactor the driver probe so that we can
supply source clock from ACPI part of the driver to the core.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>