Before assigning returned setup structure check if not null

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