i2c:designware Turn off the ctrl when setting the speed

The designware i2c controller must be turned off before
setting the speed in IC_CON register, as stated in the
section 6.3.1 of the dw_apb_i2c_db.pdf.

Signed-off-by: Michel Sanches <michel.sanches@st.com>
Signed-off-by: Armando Visconti <armando.visconti@st.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>