powercap: RAPL: Fix a NULL vs IS_ERR() bug

The devm_ioremap_resource() function returns error pointers on error,
it never returns NULL.  Update the check accordingly.

Fixes: 9eef7f9d ("powercap: intel_rapl: Introduce RAPL TPMI interface driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>