bluetooth-fw/nimble: fix discovery stop race causing KernelBG hang #1595

Job	Run time
changes-qemu-sdkshell	3s
build-qemu-sdkshell (qemu_gabbro)	5m 6s
build-qemu-sdkshell (qemu_flint)	4m 49s
build-qemu-sdkshell (qemu_emery)	4m 55s
build-qemu-sdkshell-status	2s
	14m 55s

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