ACPI: check a return value correctly in acpi_power_get_context()

We should check *resource != NULL rather than resource != NULL, which will be
always true.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>