wifi: cfg80211: Add utility for converting op_class into chandef

This utility is used in STA CSA handling. The op_class in the ECSA
Element can be converted into chandef.

Co-developed-by: Money Wang <money.wang@mediatek.com>
Signed-off-by: Michael-CY Lee <michael-cy.lee@mediatek.com>
Link: https://msgid.link/20231222010914.6521-2-michael-cy.lee@mediatek.com


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