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>