p54pci: fix bugs in p54p_check_tx_ring

Hans de Goede identified a bug in p54p_check_tx_ring:

there are two ring indices. 1 => tx data and 3 => tx management.
But the old code had a constant "1" and this resulted in spurious
dma unmapping failures.

Cc: stable@kernel.org
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=583623
Bug-Identified-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/p54/p54pci.c

@@ -245,7 +245,7 @@ static void p54p_check_tx_ring(struct ieee80211_hw *dev, u32 *index,
 	u32 idx, i;
 
 	i = (*index) % ring_limit;
-	(*index) = idx = le32_to_cpu(ring_control->device_idx[1]);
+	(*index) = idx = le32_to_cpu(ring_control->device_idx[ring_index]);
 	idx %= ring_limit;
 
 	while (i != idx) {