ath9k: Use HW HT capabilites properly

The commit "ath9k: Remove aggregation flags" changed how
nodes were being initialized. Use the HW HT cap bits
to initialize/de-initialize nodes, else we would be
accessing an uninitialized entry during a suspend/resume cycle,
resulting in a panic.

Reported-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>