STM32: change spi error to debug warning
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).

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os