drivers: spi: atcspi200: Fix the bug for inconsistent TX/RX lengths.

1. Due to the hardware limitations of the ATCSPI200 IP, the transmission
   and reception counts must be consistent. If there is a mismatch in
   length, dummy data needs to be transmitted or received.
   Under this limitation, using the spi_context_xx related APIs to
   determine whether the transmission is complete may cause issues in
   certain situations.
   Therefore, additional variables are set to record the exact number of
   bytes need to be transmitted or received.
2. Also, fix some inaccuracies in configuring DMA transfers.

Signed-off-by: Kevin Wang <kevinwang821020@google.com>