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: NBernhard Messerklinger <bernhard.messerklinger@br-automation.com>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NJagan Teki <jagan@amarulasolutions.com>

drivers/spi/spi-mem.c

@@ -214,7 +214,7 @@ int spi_mem_exec_op(struct spi_slave *slave, const struct spi_mem_op *op)
 	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