iwlwifi: mvm: remove useless check for mvm->cfg in iwl_parse_nvm_section()

At this point we have already copied the cfg pointer to mvm and we
have been dereferencing this pointer many times before, so it will
never be NULL or we would have crashed.  Remove the useless check.

Signed-off-by: Luca Coelho <luciano.coelho@intel.com>