ath9k: remove pointless sc_txintrperiod and spin_lock_bh on tx prepare

sc_txintrperiod is currently set to 0 and never updated. We won't
be using this, if anything we will consider using TX interrupt
mitigation but that is different and not yet tested. So remove
sc_txintrperiod and the pointless spin_lock_bh() on tx prepare.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath9k/ath9k.h b/drivers/net/wireless/ath9k/ath9k.h
index accace5..a6063de 100644
--- a/drivers/net/wireless/ath9k/ath9k.h
+++ b/drivers/net/wireless/ath9k/ath9k.h
@@ -138,6 +138,19 @@
 #define ATH9K_TXDESC_NOACK		0x0002
 #define ATH9K_TXDESC_RTSENA		0x0004
 #define ATH9K_TXDESC_CTSENA		0x0008
+/* ATH9K_TXDESC_INTREQ forces a tx interrupt to be generated for
+ * the descriptor its marked on.  We take a tx interrupt to reap
+ * descriptors when the h/w hits an EOL condition or
+ * when the descriptor is specifically marked to generate
+ * an interrupt with this flag. Descriptors should be
+ * marked periodically to insure timely replenishing of the
+ * supply needed for sending frames. Defering interrupts
+ * reduces system load and potentially allows more concurrent
+ * work to be done but if done to aggressively can cause
+ * senders to backup. When the hardware queue is left too
+ * large rate control information may also be too out of
+ * date. An Alternative for this is TX interrupt mitigation
+ * but this needs more testing. */
 #define ATH9K_TXDESC_INTREQ		0x0010
 #define ATH9K_TXDESC_VEOL		0x0020
 #define ATH9K_TXDESC_EXT_ONLY		0x0040