i2c: stm32f7: compute i2cclk only one time

Compute i2cclk only one time in stm32_i2c_compute_timing()
and remove setup parameter (accessible in i2c_priv).

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>