wifi: cfg80211: wext: validate chandef in monitor mode

wifi: cfg80211: wext: validate chandef in monitor mode

cfg80211_wext_siwfreq() constructs a channel definition for monitor
mode but passes it to cfg80211_set_monitor_channel() without first
validating it with cfg80211_chandef_valid(). This causes a WARN_ON
in cfg80211_chandef_dfs_required() when it receives an invalid chandef.

Add the missing cfg80211_chandef_valid() check before calling
cfg80211_set_monitor_channel() to return -EINVAL early on invalid
channel definitions, consistent with how other callers handle this.

Reported-by: syzbot+02a1a03b8622d3c7d1c9@syzkaller.appspotmail.com
Signed-off-by: Kartik Nair <contact.kartikn@gmail.com>
Link: https://patch.msgid.link/20260510202437.7857-1-contact.kartikn@gmail.com
[clarify subject]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>