ath9k_hw: do not enable the MIB interrupt in the interrupt mask register

The interrupt is no longer handling it. While it shouldn't fire (wraparound
is highly unlikely), the consequences would be fatal (interrupt storm).
Disable the interrupt to prevent that from happening.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>