rt2800: do not crash if spec->channels is NULL

In case the spec->channels was not specified, print warning instead
of hard crash the kernel.
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -5213,6 +5213,9 @@ static int rt2800_probe_hw_mode(struct rt2x00_dev *rt2x00dev)
 		spec->channels = rf_vals_3x;
 	}
 
+	if (WARN_ON_ONCE(!spec->channels))
+		return -ENODEV;
+
 	/*
 	 * Initialize HT information.
 	 */