ath5k: reduce interrupt load caused by rx/tx interrupts

While the rx/tx tasklet is pending, new unnecessary interrupts may arrive.
Decrease the load by temporarily disabling the interrupts until the tasklet
has completed.

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