classmate-laptop: should check for NULL as retval for rfkill_alloc

rfkill_alloc returns NULL when it fails if RFKILL is enabled. When RFKILL is
disabled, its return value of ERR_PTR(-ENODEV) is OK to use as all rfkill
functions will work with it, as they are simply empty stubs.
Reported-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: platform-driver-x86@vger.kernel.org
Cc: mjg@redhat.com
Cc: don@syst.com.br
Cc: rpurdie@rpsys.net
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/platform/x86/classmate-laptop.c

@@ -573,16 +573,17 @@ static int cmpc_ipml_add(struct acpi_device *acpi)
 
 	ipml->rf = rfkill_alloc("cmpc_rfkill", &acpi->dev, RFKILL_TYPE_WLAN,
 				&cmpc_rfkill_ops, acpi->handle);
-	/* rfkill_alloc may fail if RFKILL is disabled. We should still work
-	 * anyway. */
-	if (!IS_ERR(ipml->rf)) {
+	/*
+	 * If RFKILL is disabled, rfkill_alloc will return ERR_PTR(-ENODEV).
+	 * This is OK, however, since all other uses of the device will not
+	 * derefence it.
+	 */
+	if (ipml->rf) {
 		retval = rfkill_register(ipml->rf);
 		if (retval) {
 			rfkill_destroy(ipml->rf);
 			ipml->rf = NULL;
 		}
-	} else {
-		ipml->rf = NULL;
 	}
 
 	dev_set_drvdata(&acpi->dev, ipml);