drivers/spi: Handle the case when tx buf/len is NULL/>0 in DW driver

This is a rare but valid case when you request the controller to send
dummy bytes on your behalf (it saves memory as you don't need to provide
a valid buffer, just NULL and the numbers of dummy bytes as length).

Without it, user code can get stuck into interrupt loop.

Signed-off-by: Tomasz Bursztyka <[email protected]>

Author: Tomasz Bursztyka

drivers/spi/spi_dw.c

@@ -125,6 +125,8 @@ static void push_data(struct device *dev)
 				break;
 			}
 
+			data = 0;
+		} else if (spi_context_tx_on(&spi->ctx)) {
 			data = 0;
 		} else {
 			/* Nothing to push anymore */