i2c: i2c-stm32f4: use generic definition of speed enum

This patch uses a more generic definition of speed enum for i2c-stm32f4
driver.

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Reviewed-by: Ludovic BARRE <ludovic.barre@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>