wifi: cfg80211: Prevent cfg80211_wext_siwencodeext() on MLD

Currently, MLO support is not added for WEXT code and WEXT handlers are
prevented on MLDs. Prevent WEXT handler cfg80211_wext_siwencodeext()
also on MLD which is missed in commit 7b0a0e3c

 ("wifi: cfg80211: do
some rework towards MLO link APIs")

Signed-off-by: Veerendranath Jakkam <quic_vjakkam@quicinc.com>
Link: https://lore.kernel.org/r/20220730052643.1959111-3-quic_vjakkam@quicinc.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>