serial: stm32: Fix warnings when compiling with W=1

This patch solves the following warnings:

drivers/serial/serial_stm32.c: In function 'stm32_serial_probe':
warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  if (plat->clock_rate < 0) {
                       ^

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
1 file changed