i2c: stm32f7_i2c: fix data abort

As "v" is a local variable in stm32_i2c_choose_solution()
"v" has to be copied into "s" to avoid data abort in
stm32_i2c_compute_timing().

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>