spi: Always check complete callback before calling it

Since commit 1e25cd4729bd "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>
diff --git a/drivers/spi/spi-fsl-espi.c b/drivers/spi/spi-fsl-espi.c
index 428dc7a..e8e3d34 100644
--- a/drivers/spi/spi-fsl-espi.c
+++ b/drivers/spi/spi-fsl-espi.c
@@ -446,7 +446,8 @@
 
 	m->actual_length = espi_trans.actual_length;
 	m->status = espi_trans.status;
-	m->complete(m->context);
+	if (m->complete)
+		m->complete(m->context);
 }
 
 static int fsl_espi_setup(struct spi_device *spi)