AP_Mission: Change the IF statement to a SWITCH statement

[muramura]

Multiple decisions are being made for the same variable value.
In this case, the SWITCH statement is better than the IF statement.

[peterbarker]

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