ath9k: enable dynack using set_coverage_class codepath

Enable ACK timeout estimation algorithm if set_coverage_class routine has been
called with negative coverage_class parameter. Dynack is automatically disabled
setting valid value for coverage class

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>