spi: Always check complete callback before calling it

Since commit 1e25cd4729 "spi: Do not require a completion", this checking is
required to prevent NULL pointer dereference.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-fsl-spi.c

@@ -404,7 +404,8 @@ static void fsl_spi_do_one_msg(struct spi_message *m)
 	}
 
 	m->status = status;
-	m->complete(m->context);
+	if (m->complete)
+		m->complete(m->context);
 
 	if (status || !cs_change) {
 		ndelay(nsecs);