wifi: mac80211: use cfg80211_chandef_get_width()

We can now use this helper here and simplify some code.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250311122534.0a1d24a1a763.I51a52a67587a7eee65c80b9c5cf132820ebb9dd9@changeid


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