spi: ti-qspi: Make sure res_mmap != NULL before dereferencing it

resource_size() is dereferencing the res without checking that it is
not NULL, so we need to do the check before calling resource_size().

Fixes: b95cb394 ("spi: ti-qspi: Implement the spi_mem interface")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-ti-qspi.c

@@ -718,6 +718,8 @@ static int ti_qspi_probe(struct platform_device *pdev)
 				"memory mapped resource not required\n");
 		}
 	}
+
+	if (res_mmap)
 		qspi->mmap_size = resource_size(res_mmap);
 
 	irq = platform_get_irq(pdev, 0);