rt2x00: Use pretbtt irq for fetching beacons on rt2800pci

Updating the beacon on pre tbtt instead of beacondone allows much lower
latency in regard to TIM updates. Hence, use the pre tbtt interrupt for
updating the beacon in rt2800pci (older devices don't provide a pre tbtt
interrupt).

Also, add a new driver flag to indicate if a driver has pre tbtt support
or not and implement the according behavior in rt2x00lib.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2x00.h b/drivers/net/wireless/rt2x00/rt2x00.h
index 0fa2141..9dd0d17 100644
--- a/drivers/net/wireless/rt2x00/rt2x00.h
+++ b/drivers/net/wireless/rt2x00/rt2x00.h
@@ -658,6 +658,7 @@
 	CONFIG_SUPPORT_HW_CRYPTO,
 	DRIVER_SUPPORT_CONTROL_FILTERS,
 	DRIVER_SUPPORT_CONTROL_FILTER_PSPOLL,
+	DRIVER_SUPPORT_PRE_TBTT_INTERRUPT,
 	DRIVER_SUPPORT_LINK_TUNING,
 	DRIVER_SUPPORT_WATCHDOG,
 
@@ -1071,6 +1072,7 @@
  * Interrupt context handlers.
  */
 void rt2x00lib_beacondone(struct rt2x00_dev *rt2x00dev);
+void rt2x00lib_pretbtt(struct rt2x00_dev *rt2x00dev);
 void rt2x00lib_txdone(struct queue_entry *entry,
 		      struct txdone_entry_desc *txdesc);
 void rt2x00lib_rxdone(struct rt2x00_dev *rt2x00dev,