Improve frequency setting proc of Renesas SPI driver

If the specified frequency exceeds the upper limit and lower limit at SPI driver, an error is output, but this does not match policy with other renesas drivers. Thus I revises the processing as follows.
- If "hz" is maximum over, it is rounded by the maximum value.
- If "hz" is minimum under, it is rounded by the minimum value.

Author: TomoYamanaka

targets/TARGET_RENESAS/TARGET_RZ_A1XX/spi_api.c

@@ -149,9 +149,11 @@ void spi_frequency(spi_t *obj, int hz) {
 
     hz_min = pclk_base / 2 / 256 / 8;
     hz_max = pclk_base / 2;
-    if (((uint32_t)hz < hz_min) || ((uint32_t)hz > hz_max)) {
-        error("Couldn't setup requested SPI frequency");
-        return;
+    if ((uint32_t)hz < hz_min) {
+        hz = hz_min;
+    }
+    if ((uint32_t)hz > hz_max) {
+        hz = hz_max;
     }
 
     div = (pclk_base / hz / 2);