[net80211] decode WPA cipher config '0' as "clear cipher config".

wpa_supplicant actually calls the wpa ioctl with cipher 0 as part
of the teardown process and this returns an ioctl error.

It's required as part of the (hopefully!) upcoming encrypted
IBSS support which does indeed do the above as part of interface
setup and then exits ungracefully when it gets an ioctl error.

(I'll fix wpa_supplicant in a later commit as part of other work.)