iwx: migrate to new net80211 encryption key API
Migrate to the new encryption key API rather than poking at the
key struct directly.
Locally tested:
- STA mode - iwx0: <Wi-Fi 6 AX210> mem 0x84c00000-0x84c03fff at device 0.0 on pci4
Differential Revision: https://reviews.freebsd.org/D54463