luci-mod-network: Offer ft-over-air by default for 802.11r

OpenWrt commit 2984a0420 changed the default 802.11r Fast Transition
method to be ft-over-air instead of ft-over-ds.

Offer ft-over-air as the first item in the drop-down list, so that
it gets selected by default when 802.11r option is enabled.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>