i2c: stm32f7: prevent array underflow in stm32f7_get_lower_rate()

We want to break with "i" set to zero whether we find the rate we want
or not.  In the current code, if we don't find the rate we want then it
exits the loop with "i" set to -1 and results in an array underflow.

Fixes: 09cc9a3b ("i2c: stm32f7: allows for any bus frequency")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alain Volmat <alain.volmat@st.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>