ath9k: remove ah->mask_reg, it's never used properly

ah->mask_reg was used to hold different data throughout the driver.
ath9k_hw_init_interrupt_masks() used it to save the value written to
AR_IMR.  ath9k_hw_set_interrupts() used it to hold the interrupt mask as
defined in enum ath9k_int.  Those masks differ in many bits.

Use ah->imask instead of ah->mask_reg in ath9k_hw_set_interrupts() and
ath9k_hw_updatetxtriglevel().  That's what the code was meant to do.
ah->imask is initialized in ath9k_start(), so we don't need to
initialize it from ah->mask_reg.

Once it's done, ah->mask_reg becomes write-only, so it's replaced with a
local variable in ath9k_hw_init_interrupt_masks().

Signed-off-by: Pavel Roskin <proski@gnu.org>
Reported-by: Julia Lawall <julia@diku.dk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath9k/mac.c b/drivers/net/wireless/ath/ath9k/mac.c
index e020b82..4a2060e 100644
--- a/drivers/net/wireless/ath/ath9k/mac.c
+++ b/drivers/net/wireless/ath/ath9k/mac.c
@@ -105,7 +105,7 @@
 	if (ah->tx_trig_level >= ah->config.max_txtrig_level)
 		return false;
 
-	omask = ath9k_hw_set_interrupts(ah, ah->mask_reg & ~ATH9K_INT_GLOBAL);
+	omask = ath9k_hw_set_interrupts(ah, ah->imask & ~ATH9K_INT_GLOBAL);
 
 	txcfg = REG_READ(ah, AR_TXCFG);
 	curLevel = MS(txcfg, AR_FTRIG);