spi: stm32: stability & performance enhancements

Merge series from Alain Volmat <alain.volmat@foss.st.com>:

The series fixes a stability issue when dealing with <8bpw
transfers, as well as enforce an error if the DMA information
provided within the DT are incorrect.

Performance enhancement is also provided by allowing a polling
mode which is triggered when the transfer is so short that
polling mode transfer would lead to faster transfer than
if it was done in a interrupt driven manner.