mwifiex: fix NULL pointer dereference error

In mwifiex_enable_hs, we need to check if
priv->wdev.wiphy->wowlan_config is NULL before accessing its member.
This sometimes cause kernel panic when suspend/resume.

Signed-off-by: Wei-Ning Huang <wnhuang@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>