Correct off-by-one problem in EKF IMU ringbuffer #37353

Job	Run time
build (stm32f7, chibios, 10)	3m 45s
build (stm32h7, chibios, 10)	3m 43s
build (fmuv2-plane, chibios, 10)	1m 55s
build (periph-build, chibios, 10)	3m 38s
build (iofirmware, chibios, 10)	1m 21s
build (CubeOrange-bootloader, chibios, 10)	50s
build (fmuv3-bootloader, chibios, 10)	1m 18s
build (revo-bootloader, chibios, 10)	55s
build (stm32h7-debug, chibios, 10)	2m 10s
build (fmuv3, chibios, 10)	5m 39s
build (revo-mini, chibios, 10)	3m 52s
build (MatekF405-Wing, chibios, 10)	4m 39s
build (CubeOrange-ODID, chibios, 10)	2m 53s
build (CubeRedPrimary-bootloader, chibios, 10)	49s
build (configure-all, chibios, 10)	10m 11s
build (build-options-defaults-test, chibios, 10)	7m 26s
build (signing, chibios, 10)	2m 37s
build (CubeOrange-PPP, chibios, 10)	2m 9s
build (SOHW, chibios, 10)	3m 16s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 31s
	1h 4m 37s

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