drivers: spi: replace timeout for STM32 DMA slave mode

Replace the timeout for a SPI transceive in slave mode for STM32 DMA
operations with a K_FOREVER. Being an SPI slave means we do not
know when the transaction will start, hence it does not make sense
to have a timeout in such a case.
This will resolve issue #60000.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>