axi_dacfifo: Add missing read-enable signal to ad_mem instance

Commit bfc8ec28c3 ("util_axis_fifo: instantiate block ram in async mode")
added the read-enable (reb) signal to the ad_mem block.

It didn't update the ad_mem instance in axi_dacfifo_address_buffer.v. This
results in the read-enable of the address_buffer being tied to 0.

Fix this by connecting the same signal that increments the read address.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

library/xilinx/axi_dacfifo/axi_dacfifo_address_buffer.v

@@ -71,6 +71,7 @@ module axi_dacfifo_address_buffer #(
     .addra (waddr),
     .dina (din),
     .clkb (clk),
+    .reb (rea),
     .addrb (raddr),
     .doutb (dout));