AP_OSD: don't go via RCMap singleton to get RPTY RC channels

[peterbarker]

Tested in SITL using --osd

Extracted from #28089

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
Durandal                            *      *           -8      -8                -8     -8     *
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     *      *           0       0                 0      0      *
MatekF405                           *      *           0       0                 0      0      *
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           *      *           0       0                 0      0      *
skyviper-v2450                                         *                                       

[tridge]

i'd prefer const auto *chan = &chanref;

[peterbarker]

Done