jesd204: jesd204_up_common: Add missing core_cfg_transfer_en declaration

Make sure the core_cfg_transfer_en signal is declared before they are used.
Strictly speaking the current code is correct and synthesis correctly, but
declaring the signals make the intentions of the code more explicit.

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

library/jesd204/axi_jesd204_common/jesd204_up_common.v

@@ -103,6 +103,7 @@ reg [1:0] up_reset_synchronizer_vector = 2'b11;
 assign up_reset_synchronizer = up_reset_synchronizer_vector[0];
 
 /* Transfer two cycles before the core comes out of reset */
+wire core_cfg_transfer_en;
 assign core_cfg_transfer_en = core_reset_vector[2] ^ core_reset_vector[1];
 
 reg up_reset_core = 1'b1;