[iwm] Improve reliability of iwm_release on disassociation a bit.

• We need to first call ivp->iv_newstate(), to enqueue the deauth/deassoc mgmt frame, then flush the tx queue, before actually calling iwm_release().

• cycling a wlan connection via wpa_cli frontend to wpa_supplicant, by issuing disconnect and reconnect commandos works quite well. (There is still an issue when disconnecting/reconnecting too quickly)

• Reassociating or roaming via wpa_supplicant is still broken.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7943