ath5k: Stop queues only for NF calibration

As far as we know, only NF calibration interferes with RX/TX so we can
leave the queues enabled for the other calibrations.

BTW: Stopping the queues is not enough for avoiding transmissions, since there
might be packets in the queue + beacons are also sent regularly! But i leave it
like this until we have a better solution (stopping TX DMA?).

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>