ec/google/chromeec/acpi: Fix long battery string reporting for Windows

`ToString(byte x)` is undefined behavior per the ACPI spec, which
causes Windows to discard the battery device status entirely.

Fix this and improve performance of the BRSX method by using an array
to store the characters read, calling ToString() only once at the end.

TEST=build/boot Win11 and Linux on google/rex, verify battery status
reported properly under both OSes.

Change-Id: I4e5aea3b2763a3c4433abe95c3a41d218fcd06c1
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89602
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Nicholas Sudsgaard <devel+coreboot@nsudsgaard.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>

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

@@ -152,13 +152,19 @@ Method(BRSX, 1, Serialized)
 		// Read to end of string, or up to a reasonable maximum length. Reads of
 		// BSRF consume bytes from the FIFO, so take care to read it only once
 		// per byte of data.
-		Local0 = ""
-		Local1 = BSRF
-		While (Local1 != 0 && SizeOf (Local0) < 32)
+		Local0 = Buffer (33){}
+		For (Local2 = 0, Local2 < SizeOf(Local0) - 1, Local2++)
 		{
-			Local0 = Concatenate (Local0, ToString (Local1))
 			Local1 = BSRF
+			If (Local1 == 0)
+			{
+				Break
+			}
+			Local0[Local2] = Local1
 		}
+
+		Local0[Local2] = 0
+		Local0 = ToString(Local0)
 	}
 
 	// Store the result in the cache