GCS_MAVLink: respect txbuf flow control for FTP messages #37681

Job	Run time
build (stm32f7, chibios, 10)	9m 4s
build (stm32h7, chibios, 10)	7m 56s
build (fmuv2-plane, chibios, 10)	3m 26s
build (periph-build, chibios, 10)	5m 57s
build (iofirmware, chibios, 10)	1m 48s
build (CubeOrange-bootloader, chibios, 10)	52s
build (fmuv3-bootloader, chibios, 10)	48s
build (revo-bootloader, chibios, 10)	56s
build (stm32h7-debug, chibios, 10)	4m 27s
build (fmuv3, chibios, 10)	10m 37s
build (revo-mini, chibios, 10)	7m 54s
build (MatekF405-Wing, chibios, 10)	8m 26s
build (CubeOrange-ODID, chibios, 10)	6m 1s
build (CubeRedPrimary-bootloader, chibios, 10)	51s
build (configure-all, chibios, 10)	10m 11s
build (build-options-defaults-test, chibios, 10)	19m 4s
build (signing, chibios, 10)	5m 13s
build (CubeOrange-PPP, chibios, 10)	4m 39s
build (SOHW, chibios, 10)	9m 20s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 34s
	1h 59m 4s

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