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>