selftests/resctrl: Do not store iMC counter value in counter config structure

selftests/resctrl: Do not store iMC counter value in counter config structure

The MBM and MBA tests compare MBM memory bandwidth measurements against
the memory bandwidth event values obtained from each memory controller's
PMU. The memory bandwidth event settings are discovered from the memory
controller details found in /sys/bus/event_source/devices/uncore_imc_N and
stored in struct imc_counter_config.

In addition to event settings struct imc_counter_config contains
imc_counter_config::return_value in which the associated event value is
stored on every read.

The event value is consumed and immediately recorded at regular intervals.
The stored value is never consumed afterwards, making its storage as part
of event configuration unnecessary.

Remove the return_value member from struct imc_counter_config. Instead
just use a more aptly named "measurement" local variable for use during
event reading.

Link: https://lore.kernel.org/r/e0b6ad2755e2fd802f54b0bc07eeb90247baca19.1775266384.git.reinette.chatre@intel.com
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>