lenovo t400: Fix IASL warning and remark

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

Presumably, the ATPX would not need a return value if Arg0 is anything
other than 0, so just return a zero.

- Serialize ATPX method to make IASL happy.  This means that it can
only be used by one thread at a time.

Fixes these issues:
dsdt.aml   2581: Method (ATPX, 2, NotSerialized) {
Remark   2120 -            ^ Control Method should be made Serialized
(due to creation of named objects within)
dsdt.aml 2581: Method (ATPX, 2, NotSerialized) {
Warning  3115 -          ^ Not all control paths return a value (ATPX)

Change-Id: I14aeab0cebe4596e06a17cffc36cc01b953d7191
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12518
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/lenovo/t400/Kconfig

@@ -51,8 +51,4 @@ config CBFS_SIZE
 	hex
 	default 0x200000
 
-# TODO: Remove this when platform ASL is fixed
-config IASL_WARNINGS_ARE_ERRORS
-	def_bool n
-
 endif # BOARD_LENOVO_T400

src/mainboard/lenovo/t400/acpi/graphics.asl

@@ -55,7 +55,7 @@ Method(SHYB, 1) {
 	}
 }
 
-Method (ATPX, 2, NotSerialized) {
+Method (ATPX, 2, Serialized) {
 	/* Create local variables */
 	Name (ATPR, Buffer (0x08) {
 		0x0, 0x0, 0x0, 0x0,
@@ -94,4 +94,5 @@ Method (ATPX, 2, NotSerialized) {
 			SHYB(0x00)
 		}
 	}
+	Return(Zero)
 }