[iwm] Free rx ring on detach. Free nvm_sections data after parsing.

[imre_vdsz.com]

Summary

• Call iwm_free_rx_ring() when detaching.

• Free nvm_sections[i].data allocations after parsing the nvm data. As a workaround to make it build, had to remove const specifier from struct nvm_section's data value.

Obtained from: DragonFlyBSD 4dc3b92f92c7a2d9582ca7cbd06a5582d54d61be

Test Plan

Tested on AC 7260, loading and unloading the iwm module multiple times,
and using wpa_supplicant each time to connect to an AP and use the wlan
connection a little bit.

[adrian]

thanks! I'll try to get this into -head before 11.0 is cut.