Allow compass mag cal command to come in as COMMAND_INT

[peterbarker]

+ column -ts, /tmp/some.csv
Board               AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
Durandal                       -88    *           -88     -88               -88    -88    -96
HerePro             *                                                                     
Hitec-Airspeed      *                                                                     
KakuteH7-bdshot                -40    *           -32     -40               -40    -32    -32
MatekF405                      -8     *           -16     -8                -16    -8     -8
Pixhawk1-1M-bdshot             -16                -16     -16               -16    -16    -16
f103-QiotekPeriph   *                                                                     
f303-Universal      *                                                                     
iomcu                                                           *                         
revo-mini                      0      *           -8      -8                -8     8      0
skyviper-v2450                                    -16                                     

Tested in SITL + MAVProxy. We also test this in autotest.