ath6kl: Fix a suspend/resume crash in AR6004 USB

cfg80211 suspend/resume callbacks are not yet implemented
for AR6004 USB. Introduce dummy handlers for these to avoid
NULL pointer dereference.

Cc: Sivanesan Rajapupathi <c_srajap@qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>