perf/amd/ibs: Fix ->config to sample period calculation for OP PMU

Instead of using standard perf_event_attr->freq=0 and ->sample_period
fields, IBS event in 'sample period mode' can also be opened by setting
period value directly in perf_event_attr->config in a MaxCnt bit-field
format.

IBS OP MaxCnt bits are defined as:

  (high bits) IbsOpCtl[26:20] = IbsOpMaxCnt[26:20]
  (low bits)  IbsOpCtl[15:0]  = IbsOpMaxCnt[19:4]

Perf event sample period can be derived from MaxCnt bits as:

  sample_period = (high bits) | ((low_bits) << 4);

However, current code just masks MaxCnt bits and shifts all of them,
including high bits, which is incorrect. Fix it.

Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/20250115054438.1021-4-ravi.bangoria@amd.com