x86, setup: Fix earlyprintk=serial,ttyS0,115200

Torsten reported that there is garbage output,
after commit 8fee13a4 (x86,
setup: enable early console output from the decompressor)

It turns out we missed the offset for that case.
Reported-by: Torsten Kaiser <just.for.lkml@googlemail.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <4C7B0578.8090807@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/boot/early_serial_console.c

@@ -58,7 +58,7 @@ static void parse_earlyprintk(void)
 		if (arg[pos] == ',')
 			pos++;
 
-		if (!strncmp(arg, "ttyS", 4)) {
+		if (!strncmp(arg + pos, "ttyS", 4)) {
 			static const int bases[] = { 0x3f8, 0x2f8 };
 			int idx = 0;