drivers: spi_ll_stm32: refactor DMA support

- Fix GPIO CS timing when using DMA. When using GPIO CS the
  CS select was enabled after the DMA started the transfer,
  resulting in the first few bits being transfered while
  CS was still disabled.

- Fix TX or RX only DMA transfers. When only a RX or only
  a TX transfer was requested the DMA never finished.

  For the RX only cause the size on the transfer was
  calculated by taking the TX buffer length (0), this
  caused problems.

  For the TX only transfer the RX buffer was set to NULL,
  this caused the DMA to acctually writing data to the
  adress 0x00000000. By using the dummy destination buffer
  it now only writes to valid memory.

- Add semaphore to signal that DMA is ready, instead of
  just busy waiting.

Signed-off-by: Erwin Rol <erwin@erwinrol.com>