ec/google/chromeec: Update Tablet event call

The tablet event handler method TPET is defined as an extern, then
defined again in skylake, the only platform that supports it.  In newer
versions of IASL, this generates an error, as the method is defined in
two places.

Remove the extern and the CondRefOf check.  That's not needed if we
only set the EC_ENABLE_TABLET_EVENT define on platforms that have a
TPET handler.

Change-Id: I8bee069fc95637446593dfaaae1254e931421517
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/25983
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Sumeet R Pawnikar <sumeet.r.pawnikar@intel.com>

src/ec/google/chromeec/acpi/ec.asl

@@ -24,7 +24,7 @@ External (\_TZ.THRT, MethodObj)
 #ifdef DPTF_ENABLE_CHARGER
 External (\_SB.DPTF.TCHG, DeviceObj)
 #endif
-External (\_SB.DPTF.TPET, MethodObj)
+
 
 Device (EC0)
 {
@@ -373,9 +373,7 @@ Device (EC0)
 		Store ("EC: TABLET mode switch Event", Debug)
 		Notify (CREC, 0x2)
 #ifdef EC_ENABLE_TABLET_EVENT
-		If (CondRefOf (\_SB.DPTF.TPET)) {
 		\_SB.DPTF.TPET()
-		}
 #endif
 #ifdef EC_ENABLE_TBMC_DEVICE
 		Notify (TBMC, 0x80)