[efr32] implement otPlatRadioEnergyScan API (#3631)
[efr32] implement otPlatRadioEnergyScan API (#3631)

* refactor for energy scan API

* rename enum, init status in init func, tidy up and fix issue in GetRssi

* add cast and fix start energy scan error return

openthread/openthread,lanyuwen/openthread,jwhui/openthread,lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,srickardti/openthread,abtink/openthread,librasungirl/openthread,bukepo/openthread,librasungirl/openthread,jwhui/openthread,abtink/openthread,turon/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,abtink/openthread,librasungirl/openthread,bukepo/openthread,turon/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,chshu/openthread,lanyuwen/openthread,turon/openthread,chshu/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,turon/openthread,bukepo/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,bukepo/openthread,lanyuwen/openthread