nl80211: fix channel switch parsing

The nl80211 attribute NL80211_ATTR_CSA_C_OFF_BEACON should be nested
inside NL80211_ATTR_CSA_IES, but commit ee4bc9e7
("nl80211: enable IBSS support for channel switch announcements")
added a check in the outer message attributes.

Fix channel switch calls by removing the erroneus condition.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>