AP_KDECAN: standardize on 32 bit microsecond CAN timeouts

[tpwrules]

For consistency with other parts of the code.

Turns out there is no compiler output change (unless I change a timeout value); the compiler already does this optimization. But I already wrote the code and spent the CPU to test it... The eventual goal is to make CANSensor::write_frame take a 32 bit value.

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

[peterbarker]

Can be merged once surprising binary-identical-output is confirmed