wifi: iwlwifi: mvm: refactor duplicate chanctx condition

Refactor the check for using a chanctx's def vs. min_def,
to have the same in both places and reuse it later.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240129211905.6fcde4051adf.I343934874612d21727ed167accaa967958b2c25b@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>