rockchip: spi: only wait for completion, if transmitting

The logic in the main transmit loop took a bit of reading the TRM to
fully understand (due to silent assumptions based in internal logic):
the "wait until idle" at the end of each iteration through the loop is
required for the transmit-path as each clearing of the ENA register
(to update run-length in the CTRLR1 register) will implicitly flush
the FIFOs... transmisson can therefore not overlap loop iterations.

This change adds a comment to clarify the reason/need for waiting
until the controller becomes idle and wraps the entire check into an
'if (out)' to make it clear that this is required for transfers with a
transmit-component only (for transfers having a receive-component,
completion of the transmit-side is trivially ensured by having
received the correct number of bytes).

The change does not increase execution time measurably in any of my
tests.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

drivers/spi/rk_spi.c

@@ -379,9 +379,18 @@ static int rockchip_spi_xfer(struct udevice *dev, unsigned int bitlen,
 				toread--;
 			}
 		}
+
+		/*
+		 * In case that there's a transmit-component, we need to wait
+		 * until the control goes idle before we can disable the SPI
+		 * control logic (as this will implictly flush the FIFOs).
+		 */
+		if (out) {
 			ret = rkspi_wait_till_not_busy(regs);
 			if (ret)
 				break;
+		}
+
 		len -= todo;
 	}