[iwm] Remove dead fw_totlen from struct iwm_fw_sects; clean up struct iwm_nvm_data.

• No functional change, none of these values were ever read.

• The values removed from struct iwm_nvm_data are only used for old dvm devices in Linux iwlwifi, and irrelevant to iwm hence.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7945