powercap: intel_rapl: remove redundant store to value after multiply

There is no need to store the result of the multiply back to variable value
after the multiplication. The store is redundant, replace *= with just *.

Cleans up clang scan build warning:
warning: Although the value stored to 'value' is used in the enclosing
expression, the value is never actually read from 'value'
[deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>