wpa_supplicant: Fix mesh encryption config

wpa_supplicant allows only SAE as the key management
type for mesh mode. The recent key_mgmt rework unconditionally
added WPA-PSK - this breaks interface bringup and wpa_s
throws this error message:

Line 10: key_mgmt for mesh network should be open or SAE
Line 10: failed to parse network block.
Failed to read or parse configuration '/var/run/wpa_supplicant-wlan0.conf

Fix this by making sure that only SAE is used for mesh.

Signed-off-by: Sujith Manoharan <m.sujith@gmail.com>