perf: CXL: fix CPMU filter value mask length

CPMU filter value is described as 4B length in CXL r3.0 8.2.7.2.2.
However, it is used as 2B length in code and comments.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Hojin Nam <hj96.nam@samsung.com>
Link: https://lore.kernel.org/r/20240216014522.32321-1-hj96.nam@samsung.com


Signed-off-by: Will Deacon <will@kernel.org>