set RCx_TRIM

def cmd_rctrim(self, args):
        '''set RCx_TRIM'''
        if not 'RC_CHANNELS_RAW' in self.status.msgs:
            print("No RC_CHANNELS_RAW to trim with")
            return
        m = self.status.msgs['RC_CHANNELS_RAW']
        for ch in range(1,5):
            self.param_set('RC%u_TRIM' % ch, getattr(m, 'chan%u_raw' % ch))