ath9k: Fix max pattern check

Since the maximum number of configurable patterns
is chip-specific, use the HW capability instead
of a fixed value for checking if a free pattern
slot is available.

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