AP_RCProtocol: add separate define for AP_RCPROTOCOL_DSM_ENABLED

[peterbarker]

Durandal                       *               *      *           *       *                 *      *      *
HerePro             *                                                                                     
Hitec-Airspeed      *                                                                                     
KakuteH7-bdshot                *               *      *           *       *                 *      *      *
MatekF405                      *               *      *           *       *                 *      *      *
Pixhawk1-1M-bdshot             *               *                  *       *                 *      *      *
f103-QiotekPeriph   *                                                                                     
f303-Universal      *                                                                                     
iomcu                                                                           *                         
revo-mini                      *               *      *           *       *                 *      *      *
skyviper-v2450                                                    *