soc/intel/quark/spi.c: Explain a read in order to flush buffers

In order for this (seemingly unnecessary) status assignment to stay, let's
explain it in a comment.

Change-Id: I0a364539c37005cfd637b75c8cc23b84e274294d
Signed-off-by: Martin Kepplinger <martink@posteo.de>
Reviewed-on: https://review.coreboot.org/20411
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/intel/quark/spi.c

@@ -166,7 +166,10 @@ static int xfer(const struct spi_slave *slave, const void *dout,
 		ctrlr->address = (data[0] << 16)
 			       | (data[1] << 8)
 			       |  data[2];
+
+		/* read in order to flush the write buffer */
 		status = ctrlr->address;
+
 		data += 3;
 		bytesout -= 3;
 	}