ACPI: CPPC: Fix uninitialized ref variable in cppc_get_perf_caps()

ACPI: CPPC: Fix uninitialized ref variable in cppc_get_perf_caps()

Commit 8505bfb4e4ec ("ACPI: CPPC: Move reference performance
to capabilities") introduced a logical error when retrieving
the reference performance.

On platforms lacking the reference performance register, the fallback
logic leaves the local 'ref' variable uninitialized (0). This causes
the subsequent sanity check to incorrectly return -EFAULT, breaking
amd_pstate initialization.

Fix this by assigning 'ref = nom' in the fallback path.

Fixes: 8505bfb4e4ec ("ACPI: CPPC: Move reference performance to capabilities")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20260310003026.GA2639793@ax162/
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
[ rjw: Subject tweak ]
Link: https://patch.msgid.link/20260311071334.1494960-1-zhangpengjie2@huawei.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>