thermal: intel: powerclamp: Fix duration module parameter

After the switch to use the powercap/idle-inject framework in the Intel
powerclamp driver, the idle duration unit is microsecond.

However, the module parameter for idle duration is in milliseconds, so
convert it to microseconds in the "set" callback and back to milliseconds
in a new "get" callback.

While here, also use mutex protection for setting and getting "duration".

The other uses of "duration" are already protected by the mutex.

Fixes: 8526eb7f ("thermal: intel: powerclamp: Use powercap idle-inject feature")
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>