i7300_idle: Fix compile warning CONFIG_I7300_IDLE_IOAT_CHANNEL not defined

When I7300_idle driver is not configured, there is a compile time
warning about IDLE_IOAT_CHANNEL not defined. Fix it.

Reported-by: Suresh Siddha <suresh.b.siddha@intel.com>
Reported-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>