iwlwifi: nvm: free old section data when reading nvm file

When reading external NVM file, if a section exists both in OTP
and in the external file, the memory that was allocated at OTP
reading is not freed.
This is possible only on systems that have an external NVM
file which is typically the case on embedded systems.

Signed-off-by: Moshe Harel <moshe.harel@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>