iwlwifi: adjust NVM parsing APIs for new a000 method

In a000 devices we will get all nvm data from the firmware,
and can save most of the parsing.
Export two APIs that op mode will still use.
Adjust API of init_sbands to be independent of NVM file structure
so it can be used by op mode as well.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>