ACPICA: acpidump: Fix truncated RSDP signature validation.

This patch enforces a rule to always use ACPI_VALIDATE_RSDP_SIG for RSDP
signatures passed from table header or ACPI_SIG_RSDP so that truncated
string comparison can be avoided.  This could help to fix the issue that
"RSD " matches but "RSD PTR " doesn't match.  Lv Zheng.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>