It conforms with Reference Manual I2C timing section.

Fixes: aeb068c5 ("i2c: i2c-stm32f7: add driver")
Signed-off-by: NNicolas Le Bayon <nicolas.le.bayon@st.com>
Signed-off-by: NBich Hemon <bich.hemon@st.com>
Reviewed-by: NPierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>