fpga: enable USB core

Actually compile in the USB core.

fpga/max80.sv

@@ -588,7 +588,6 @@ module max80 (
 	    .tty_txd ( tty_data_out ) // DTE -> DCE
 	    );
 
-`ifdef USB
    max80_usb usb (
 		  .rst_n   ( rst_n ),
 		  .clock48 ( clock_48 ),
@@ -598,7 +597,6 @@ module max80 (
 		  .usb_dn  ( gpio[5] ),
 		  .usb_pu  ( gpio[1] )
 		  );
-`endif
 
    assign tty_data_in      = tty_txd;
    assign tty_rxd          = tty_data_out;