i2c: stm32f7_i2c: Fix warnings when compiling with W=1

i2c: stm32f7_i2c: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/i2c/stm32f7_i2c.c: In function 'stm32_i2c_compute_solutions':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (scldel < scldel_min)
               ^
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (((sdadel >= sdadel_min) &&
                  ^~
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
          (sdadel <= sdadel_max)) &&
                  ^~
drivers/i2c/stm32f7_i2c.c: In function 'stm32_i2c_choose_solution':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
      if (clk_error < clk_error_prev) {
                    ^
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>