ath9k_hw: Set default slottime as 9us

Initialize 9us slot time as that is what is used mostly
(for non-ERP cases) and also to be in sync with initvals.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>