ath9k: Register max WOW patterns

Since the number of patterns that can be configured in
the HW is higher for newer chips, store the chip-specific
value in ath9k_hw_wow.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>