i2c: designware_i2c: Correct the selection of speed mode

Unfortunately a recent change adjusted the order of the checks here such
that 400MHz now shows up as fast-plus speed (1Mbps). Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: d96440d1 ("i2c: designware_i2c: Add support for fast-plus speed")