dbus: Fix WPS property of fi.w1.wpa_supplicant1.BSS interface

dbus: Fix WPS property of fi.w1.wpa_supplicant1.BSS interface

The dbus interface documentation says the following about the
WPS property of the fi.w1.wpa_supplicant1.BSS interface:

==
WPS information of the BSS. Empty dictionary indicates no WPS support.
Dictionary entries are:

Type	s	"pbc", "pin", ""
==

However the implementation returns "type" => "" for BSSes
that do not support WPS.

Fix the implementation to match the documentation.
Return empty dictionary if there is no WPS support.
And "type" => "" if WPS is supported, but is not in progress
right now.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>