wifi: cfg80211: fix width unit in cfg80211_radio_chandef_valid()

wifi: cfg80211: fix width unit in cfg80211_radio_chandef_valid()

The original code used nl80211_chan_width_to_mhz(), which returns the width in MHz.
However, the expected unit is KHz.

Fixes: 510dba80ed66 ("wifi: cfg80211: add helper for checking if a chandef is valid on a radio")
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://patch.msgid.link/df54294e6c4ed0f3ceff6e818b710478ddfc62c0.1758579480.git.Ryder%20Lee%20ryder.lee@mediatek.com/
Signed-off-by: Johannes Berg <johannes.berg@intel.com>