i2c: stm32f7: Make structure stm32f7_i2c_algo constant

Static structure stm32f7_i2c_algo, of type i2c_algorithm, is used only
when it is assigned to constant field algo of a variable having type
i2c_adapter. As stm32f7_i2c_algo is therefore never modified, make it
const as well to protect it from unintended modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Acked-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>