ec/starlabs/merlin/acpi: Replace Add(a,b,c) with ASL 2.0 syntax

Replace `Add (a, b, c)` with `c = a + b`.

Change-Id: I4f8b71c90f0c56fdb29c4d2e9a2a737f704f3265
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/60493
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>

src/ec/starlabs/merlin/acpi/ec.asl

@@ -126,7 +126,7 @@ Scope (\_SB.PCI0.LPCB)
 						}
 						Sleep (1)
 						Arg1 = Arg0
-						Add (Local1, 1, Local1)
+						Local1 += 1
 						If (Local1 == 0x03) {
 							Break
 						}