spi: Fix potential NULL pointer dereference in spi_shutdown()

Shutdown bus function might be called on the unbound device, so add a
check if there is a driver before calling its shutdown function.

This fixes following kernel panic obserbed during system reboot:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018
...
Call trace:
 spi_shutdown+0x10/0x38
 kernel_restart_prepare+0x34/0x40
 kernel_restart+0x14/0x88
 __do_sys_reboot+0x148/0x248
 __arm64_sys_reboot+0x1c/0x28
 el0_svc_common.constprop.3+0x74/0x198
 do_el0_svc+0x20/0x98
 el0_sync_handler+0x140/0x1a8
 el0_sync+0x140/0x180
Code: f9403402 d1008041 f100005f 9a9f1021 (f9400c21)
---[ end trace 266c07205a2d632e ]---

Fixes: 9db34ee6 (spi: Use bus_type functions for probe, remove and shutdown)
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/r/20201124131523.32287-1-m.szyprowski@samsung.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -426,10 +426,12 @@ static int spi_remove(struct device *dev)
 
 static void spi_shutdown(struct device *dev)
 {
-	const struct spi_driver		*sdrv = to_spi_driver(dev->driver);
+	if (dev->driver) {
+		const struct spi_driver	*sdrv = to_spi_driver(dev->driver);
 
-	if (sdrv->shutdown)
-		sdrv->shutdown(to_spi_device(dev));
+		if (sdrv->shutdown)
+			sdrv->shutdown(to_spi_device(dev));
+	}
 }
 
 struct bus_type spi_bus_type = {