iwlwifi: mvm: don't assume data section is at 0x800000

In theory, the firmware format allows changing the data section
offset. This may not be used today, but there's no reason for
the driver to assume the data section is always at 0x800000 as
it can know better.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>