spi: spi-mem: Check if exec_op function is set before calling it

Add check if exec_op is set before calling it.
At the moment it is called unconditionally, which leads to a crash if it
is not set correctly.

Signed-off-by: Bernhard Messerklinger <bernhard.messerklinger@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
index 1bb0987..b86eee7 100644
--- a/drivers/spi/spi-mem.c
+++ b/drivers/spi/spi-mem.c
@@ -214,7 +214,7 @@
 	if (ret < 0)
 		return ret;
 
-	if (ops->mem_ops) {
+	if (ops->mem_ops && ops->mem_ops->exec_op) {
 #ifndef __UBOOT__
 		/*
 		 * Flush the message queue before executing our SPI memory