i2c: i2c-stm32f7: fix no check on returned setup

Before assigning returned setup structure check if not null

Fixes: 463a9215 ("i2c: stm32f7: fix setup structure")
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org