i2c: stm32f7: fix setup structure

I2C drive setup structure is not properly allocated.
Make it static instead of pointer to store driver data.

Fixes: aeb068c5 ("i2c: i2c-stm32f7: add driver")
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>