Fixed a corner-case bug in AXI4_Fabric when num-masters = 1 (detail follows)
Fixed a corner-case bug in AXI4_Fabric when num-masters = 1 (detail follows)

The fabric uses an index (0..num_masters-1) to identify a master so it
knows how to route responses. When num_masters=1, it's identification
is a Bit#(0), on which equality tests are dicey.  Fixed to assume a
range (0..num_masters), so at least Bit #(1).

bluespec/Flute,bluespec/Flute,bluespec/Flute