[iwm] free RX ring / NVM memory after they're used.

• Free RX ring during detach
• Free NVM memory after parsing

Tested:

• 7260, STA mode

Submitted by: Imre Vadasz <imre@vdsz.com>
Approved by: re (gjb)
Obtained from: dragonflybsd
Differential Revision: https://reviews.freebsd.org/D6817