acpi/ghes: move offset calculus to a separate function

acpi/ghes: move offset calculus to a separate function

Currently, CPER address location is calculated as an offset of
the hardware_errors table. It is also badly named, as the
offset actually used is the address where the CPER data starts,
and not the beginning of the error source.

Move the logic which calculates such offset to a separate
function, in preparation for a patch that will be changing the
logic to calculate it from the HEST table.

While here, properly name the variable which stores the cper
address.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <60fdd1bf379ba1db3099710868802aa49a27febb.1736945236.git.mchehab+huawei@kernel.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>