[posix] fix otPlatAlarmMicroGetNow to use scaled time (#5070)
[posix] fix otPlatAlarmMicroGetNow to use scaled time (#5070)

This makes it consistent with MilliGetNow() counterpart.
chshu/openthread,abtink/openthread,lanyuwen/openthread,librasungirl/openthread,chshu/openthread,lanyuwen/openthread,openthread/openthread,lanyuwen/openthread,bukepo/openthread,librasungirl/openthread,bukepo/openthread,librasungirl/openthread,abtink/openthread,jwhui/openthread,lanyuwen/openthread,bukepo/openthread,chshu/openthread,jwhui/openthread,chshu/openthread,srickardti/openthread,lanyuwen/openthread,jwhui/openthread,abtink/openthread,chshu/openthread,openthread/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,abtink/openthread,librasungirl/openthread,lanyuwen/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread