mainboard/siemens/sitemp_g1p1: Fix CMOS checksum algorithm here, too

Some time ago our CMOS checksum algorithm was changed under the topic:
    Fix our CMOS checksum algorithm so it matches what /dev/nvram expects

Here is another copy of the algorithm that had to be updated.

Change-Id: I58659c7b8a89c89c76efdff405ee0620e7302277
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: http://review.coreboot.org/1852
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/siemens/sitemp_g1p1/romstage.c

@@ -147,7 +147,7 @@ static inline int cmos_chksum_valid(void) {
 #if DUMP_CMOS_RAM
 	__DEBUG__("\n");
 #endif
-	sum = (sum & 0xffff) ^ 0xffff;
+	sum = (sum & 0xffff);
 
 	/* Read the stored checksum */
 	outb(LB_CKS_LOC, 0x72);