ec/lenovo/h8: Fix IASL warnings

If any path in a method returns a value, IASL expects that all paths
within that method will return a value.

Presumably the MKHP method wouldn't get called unless there were a
pending event, but if no event is found, return a zero.

Fixes IASL warning:
dsdt.aml 1785: Method (MHKP, 0, NotSerialized)
Warning 3115 -           ^ Not all control paths return a value (MHKP)

This was the only IASL warning in most lenovo mainboards.

Change-Id: Id93dcc4a74bd4c18b78f1dde821e7ba0f3444da3
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12517
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/ec/lenovo/h8/acpi/ec.asl

@@ -300,6 +300,7 @@ Device(EC)
 		       Add (Local0, 0x5000, Local0)
 		       Return (Local0)
 		    }
+		    Return (Zero)
 	       }
 	       /* Report event  */
 	       Method (RHK, 1, NotSerialized) {