mb/google/rex: Fix ACPI MPTS method for non-5G board SKUs

MPTS method should only be generated for the board sku with 5G.

BUG=NA
TEST=Check kernel messages when going to S3. The following errors
should not be seen:
ACPI BIOS Error (bug):
	Could not resolve symbol [\_SB.PCI0.RP06.RTD3._STA]
ACPI Error:
	Aborting method \_SB.MPTS due to previous error (AE_NOT_FOUND)
ACPI Error:
	Aborting method \_PTS due to previous error (AE_NOT_FOUND)

Signed-off-by: Cliff Huang <cliff.huang@intel.com>
Change-Id: I78f434c9049773cf5229d3a1f3934ae82d1fe46d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77690
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/mainboard/google/rex/mainboard.c b/src/mainboard/google/rex/mainboard.c
index d59d3d4..619ca71 100644
--- a/src/mainboard/google/rex/mainboard.c
+++ b/src/mainboard/google/rex/mainboard.c
@@ -103,7 +103,7 @@
 {
 	const struct device *wwan = DEV_PTR(rp6_wwan);
 
-	if (wwan) {
+	if (is_dev_enabled(wwan)) {
 		acpigen_write_scope("\\_SB");
 		acpigen_write_method_serialized("MPTS", 1);
 		mainboard_generate_wwan_shutdown(wwan);