iwlwifi: mvm: support NVM file with header

Handle NVM file header. When NVM header detected, skip
the header and reading only the sections data.

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