arm64/mpam: set default feedback of last_cmd_status interface as null string

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I95IFO


CVE: NA

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Set default feedback of last_cmd_status interface as null string, in case
misunderstanding the execution result of the previous command.

Fixes: 9b3a7fd0 ("x86/intel_rdt: Add framework for better RDT UI diagnostics")
Signed-off-by: Zeng Heng <zengheng4@huawei.com>