wb_intercon: allow single master/slaves

The result of clog2(1) = 0, which will result in
master/slave_sel bit vectors of [-1:0].
Threat the special case of num_masters/slaves == 1,
so a bit vector [0:0] is created.
wb_intercon: fix multiple masters to single slave connections

The commit: 5f7cbf7e77a6d54641efc4c832628265a605d1e2
'wb_intercon: fix single slave connections'
took care of the case with a single master connected to
a single slave. The case with several masters connected
to a single slave was however oversighted.