iwlwifi: mvm: fix bug in parse_nvm_sections

The old code checks if hw_section_num is valid while the
right thing to do is to check if section[hw_section_num].data
is valid.

Signed-off-by: Eran Harary <eran.harary@intel.com>
Reviewed-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>