qcacld-3.0: Fix to accessing NULL pointer

From userspace __wlan_hdd_cfg80211_get_usable_channel got called
while WLAN is shutting down. HDD/OSIF should return failure but the
reg_get_band_channel_list API is trying to access the pdev context
from HDD which is NULL and leads to crash.

Add check in __wlan_hdd_cfg80211_get_usable_channel if the wlan
is shutdown with state DRIVER_MODULES_CLOSED and return.

Change-Id: I856075d00f3ee2ed84f4b63c33a2a8ffb85ae646
CRs-Fixed: 3541271