wifi: allow band selection in Infrastructure mode

Like GNOME nm-connection-editor, show the band selector for Infrastructure connections too. There is no reason to hide it: restricting to a specific band (2.4 GHz or 5 GHz) is just as useful when connecting to an AP as it is in Ad-hoc or Access Point mode.

Channel is not saved in Infrastructure mode since the AP determines it; saving a stale value would restrict the client to a specific channel and cause connection failures if the AP is on a different one.