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-mpc52xx-psc.c

@@ -248,7 +248,8 @@ static void mpc52xx_psc_spi_work(struct work_struct *work)
 		}
 
 		m->status = status;
-		m->complete(m->context);
+		if (m->complete)
+			m->complete(m->context);
 
 		if (status || !cs_change)
 			mpc52xx_psc_spi_deactivate_cs(spi);