Input: cros_ec_keyb - fix an invalid memory access

If cros_ec_keyb_register_matrix() isn't called (due to
`buttons_switches_only`) in cros_ec_keyb_probe(), `ckdev->idev` remains
NULL.  An invalid memory access is observed in cros_ec_keyb_process()
when receiving an EC_MKBP_EVENT_KEY_MATRIX event in cros_ec_keyb_work()
in such case.

  Unable to handle kernel read from unreadable memory at virtual address 0000000000000028
  ...
  x3 : 0000000000000000 x2 : 0000000000000000
  x1 : 0000000000000000 x0 : 0000000000000000
  Call trace:
  input_event
  cros_ec_keyb_work
  blocking_notifier_call_chain
  ec_irq_thread

It's still unknown about why the kernel receives such malformed event,
in any cases, the kernel shouldn't access `ckdev->idev` and friends if
the driver doesn't intend to initialize them.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://patch.msgid.link/20251104070310.3212712-1-tzungbi@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: Tzung-Bi Shih

drivers/input/keyboard/cros_ec_keyb.c

@@ -261,6 +261,12 @@ static int cros_ec_keyb_work(struct notifier_block *nb,
 	case EC_MKBP_EVENT_KEY_MATRIX:
 		pm_wakeup_event(ckdev->dev, 0);
 
+		if (!ckdev->idev) {
+			dev_warn_once(ckdev->dev,
+				      "Unexpected key matrix event\n");
+			return NOTIFY_OK;
+		}
+
 		if (ckdev->ec->event_size != ckdev->cols) {
 			dev_err(ckdev->dev,
 				"Discarded incomplete key matrix event.\n");