driver: spi: mcux_dspi: remove obsolete rx_bufs check

The guard against rx_bufs == NULL was originally added to prevent a
DMA misconfiguration observed when no RX buffer was provided.

The issue can no longer be reproduced, and the check was incomplete
since it did not cover rx_bufs->buffers == NULL or rx_bufs->count == 0,
which lead to the same effective condition.

Removing the check simplifies the driver and avoids blocking valid
TX-only transfers.

Signed-off-by: Alexandre Rey <alx.rey@icloud.com>