powercap: intel_rapl: Do not change CLAMPING bit if ENABLE bit cannot be changed

PL1 cannot be disabled on some platforms. The ENABLE bit is still set
after software clears it. This behavior leads to a scenario where, upon
user request to disable the Power Limit through the powercap sysfs, the
ENABLE bit remains set while the CLAMPING bit is inadvertently cleared.

According to the Intel Software Developer's Manual, the CLAMPING bit,
"When set, allows the processor to go below the OS requested P states in
order to maintain the power below specified Platform Power Limit value."

Thus this means the system may operate at higher power levels than
intended on such platforms.

Enhance the code to check ENABLE bit after writing to it, and stop
further processing if ENABLE bit cannot be changed.

Reported-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Fixes: 2d281d81 ("PowerCap: Introduce Intel RAPL power capping driver")
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Link: https://patch.msgid.link/20250619071340.384782-1-rui.zhang@intel.com


[ rjw: Use str_enabled_disabled() instead of open-coded equivalent ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>