STM32: change spi error to debug warning

In case the selected frequency is higher than the requested one, it is
better to send a debug warning rather than an blocking error.

In case of such warning, user may need to redefine the clock tree setting
at higher level (reducing peripheral's input clocks during init phase).

Author: LMESTM

targets/TARGET_STM/stm_spi_api.c

@@ -308,7 +308,7 @@ void spi_frequency(spi_t *obj, int hz) {
 
     /*  In case maximum pre-scaler still gives too high freq, raise an error */
     if (spi_hz > hz) {
-        error("Couldn't set suitable spi freq: request:%d, lowest:%d\r\n", hz, spi_hz);
+        DEBUG_PRINTF("WARNING: lowest SPI freq (%d)  higher than requested (%d)\r\n", spi_hz, hz);
     }
 
     DEBUG_PRINTF("spi_frequency, request:%d, select:%d\r\n", hz, spi_hz);