b43: Fix noise calculation WARN_ON

This removes a WARN_ON that is responsible for the following koops:
http://www.kerneloops.org/searchweek.php?search=b43_generate_noise_sample

The comment in the patch describes why it's safe to simply remove
the check.
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/b43.h

@@ -630,7 +630,6 @@ struct b43_pio {
 
 /* Context information for a noise calculation (Link Quality). */
 struct b43_noise_calculation {
-	u8 channel_at_start;
 	bool calculation_running;
 	u8 nr_samples;
 	s8 samples[8][4];

drivers/net/wireless/b43/main.c

@@ -1145,7 +1145,6 @@ static void b43_generate_noise_sample(struct b43_wldev *dev)
 	b43_jssi_write(dev, 0x7F7F7F7F);
 	b43_write32(dev, B43_MMIO_MACCMD,
 		    b43_read32(dev, B43_MMIO_MACCMD) | B43_MACCMD_BGNOISE);
-	B43_WARN_ON(dev->noisecalc.channel_at_start != dev->phy.channel);
 }
 
 static void b43_calculate_link_quality(struct b43_wldev *dev)
@@ -1154,7 +1153,6 @@ static void b43_calculate_link_quality(struct b43_wldev *dev)
 
 	if (dev->noisecalc.calculation_running)
 		return;
-	dev->noisecalc.channel_at_start = dev->phy.channel;
 	dev->noisecalc.calculation_running = 1;
 	dev->noisecalc.nr_samples = 0;
 
@@ -1171,9 +1169,16 @@ static void handle_irq_noise(struct b43_wldev *dev)
 
 	/* Bottom half of Link Quality calculation. */
 
+	/* Possible race condition: It might be possible that the user
+	 * changed to a different channel in the meantime since we
+	 * started the calculation. We ignore that fact, since it's
+	 * not really that much of a problem. The background noise is
+	 * an estimation only anyway. Slightly wrong results will get damped
+	 * by the averaging of the 8 sample rounds. Additionally the
+	 * value is shortlived. So it will be replaced by the next noise
+	 * calculation round soon. */
+
 	B43_WARN_ON(!dev->noisecalc.calculation_running);
-	if (dev->noisecalc.channel_at_start != phy->channel)
-		goto drop_calculation;
 	*((__le32 *)noise) = cpu_to_le32(b43_jssi_read(dev));
 	if (noise[0] == 0x7F || noise[1] == 0x7F ||
 	    noise[2] == 0x7F || noise[3] == 0x7F)
@@ -1214,11 +1219,10 @@ static void handle_irq_noise(struct b43_wldev *dev)
 			average -= 48;
 
 		dev->stats.link_noise = average;
-	      drop_calculation:
 		dev->noisecalc.calculation_running = 0;
 		return;
 	}
-      generate_new:
+generate_new:
 	b43_generate_noise_sample(dev);
 }