NFC: Prevent polling when device is down

Some devices turn radio on whenever they're asked to start a poll.
To prevent that from happening, we just don't call into the driver
start_poll hook when the NFC device is down.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/net/nfc/core.c b/net/nfc/core.c
index 6ceee8e..c571ca9 100644
--- a/net/nfc/core.c
+++ b/net/nfc/core.c
@@ -143,6 +143,11 @@
 		goto error;
 	}
 
+	if (!dev->dev_up) {
+		rc = -ENODEV;
+		goto error;
+	}
+
 	if (dev->polling) {
 		rc = -EBUSY;
 		goto error;