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

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

This patch solves the following warnings:

drivers/spi/stm32_spi.c: In function 'stm32_spi_write_txfifo':
drivers/spi/stm32_spi.c:116:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if (priv->tx_len >= sizeof(u32) &&
                    ^~
drivers/spi/stm32_spi.c:122:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   } else if (priv->tx_len >= sizeof(u16) &&
                           ^~
drivers/spi/stm32_spi.c: In function 'stm32_spi_read_rxfifo':
drivers/spi/stm32_spi.c:150:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       (priv->rx_len >= sizeof(u32) || (sr & SPI_SR_RXWNE))) {
                     ^~
drivers/spi/stm32_spi.c:156:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
       (priv->rx_len >= sizeof(u16) ||
                     ^~
drivers/core/simple-bus.c:15:12: warning: no previous prototype for 'simple_bus_translate' [-Wmissing-prototypes]
 fdt_addr_t simple_bus_translate(struct udevice *dev, fdt_addr_t addr)
            ^~~~~~~~~~~~~~~~~~~~
drivers/spi/stm32_spi.c: In function 'stm32_spi_set_speed':
drivers/spi/stm32_spi.c:335:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
      div > STM32_MBR_DIV_MAX)
          ^
drivers/spi/stm32_spi.c:344:19: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
  if ((mbrdiv - 1) < 0)
                   ^
drivers/spi/stm32_spi.c: In function 'stm32_spi_probe':
drivers/spi/stm32_spi.c:531:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (i = 0; i < ARRAY_SIZE(priv->cs_gpios); i++) {
                ^
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>