GCS_MAVLink: reject SET_MESSAGE_INTERVAL commands with p3 set #42000

Job	Run time
build (stm32f7, chibios, 10)	5m 51s
build (stm32h7, chibios, 10)	5m 28s
build (fmuv2-plane, chibios, 10)	3m 55s
build (periph-build, chibios, 10)	5m 54s
build (iofirmware, chibios, 10)	3m 44s
build (CubeOrange-bootloader, chibios, 10)	3m 2s
build (fmuv3-bootloader, chibios, 10)	2m 57s
build (revo-bootloader, chibios, 10)	3m 15s
build (stm32h7-debug, chibios, 10)	4m 17s
build (fmuv3, chibios, 10)	7m 33s
build (revo-mini, chibios, 10)	6m 38s
build (MatekF405-Wing, chibios, 10)	6m 46s
build (CubeOrange-ODID, chibios, 10)	4m 52s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 1s
build (configure-all, chibios, 10)	12m 55s
build (build-options-defaults-test, chibios, 10)	8m 59s
build (signing, chibios, 10)	4m 41s
build (CubeOrange-PPP, chibios, 10)	4m 7s
build (CubeOrange-EKF2, chibios, 10)	4m 26s
build (SOHW, chibios, 10)	5m 45s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 23s
build (new-check, chibios, 10)	6m 24s
	1h 57m 53s

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