wifi: cfg80211: expose cfg80211_chandef_get_width()

This can be just a trivial inline, to simplify some code.
Expose it, and also use it in util.c where it wasn't
previously available.

Reviewed-by: Miriam Rachel Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://patch.msgid.link/20250311122534.c5c3b4af9a74.Ib25cf60f634dc359961182113214e5cdc3504e9c@changeid


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