hostapd: fallback to psk when generating r0kh/r1kh

The 80211r r0kh and r1kh defaults are generated from the md5sum of
"$mobility_domain/$auth_secret".  auth_secret is only set when using EAP
authentication, but the default key is used for SAE/PSK as well.  In
this case,  auth_secret is empty, and the default value of the key can
be computed from the SSID alone.

Fallback to using $key when auth_secret is empty.  While at it, rename
the variable holding the generated key from 'key' to 'ft_key', to avoid
clobbering the PSK.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
[make ft_key local]
Signed-off-by: David Bauer <mail@david-bauer.net>