mac80211: parse wide bandwidth channel switch IE with workaround

Continuing the workaround implemented in commit 23665aaf


("mac80211: Interoperability workaround for 80+80 and 160 MHz channels")
use the same code to parse the Wide Bandwidth Channel Switch element
by converting to VHT Operation element since the spec also just refers
to that for parsing semantics, particularly with the workaround.

While at it, remove some dead code - the IEEE80211_STA_DISABLE_40MHZ
flag can never be set at this point since it's checked earlier and the
wide_bw_chansw_ie pointer is set to NULL if it's set.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>