i2c: i2c-stm32f7: make structure stm32f7_setup static const

The structure stm32f7_setup is local to the source and does not need
to be in global scope, make it static const.

Cleans up sparse warning:
symbol 'stm32f7_setup' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>