Update CST of PMOD-LED

[martinloretzzz]

The pmod-led example didn't work out of the box with a TangPrimer-25K, the pin configuration is wrong for the board. More info in the Issue #11.

[FishOfTheNorthStar]

This helped me out a lot, thanks.

This version compiled fine for me, although it did have one warning I was able to remove by changing:

generate for ( pmod_sel = 0 ; pmod_sel < pmod_num ; pmod_sel = pmod_sel + 1) 
    assign pmod_io[ pmod_sel * 8 + 7 : pmod_sel * 8 ] = ~ led_8_state_reg ;
endgenerate

to:

generate for ( pmod_sel = 0 ; pmod_sel < pmod_num ; pmod_sel = pmod_sel + 1) begin : assign_io_block
    assign pmod_io[ pmod_sel * 8 + 7 : pmod_sel * 8 ] = ~ led_8_state_reg ;
    end
endgenerate

in pmod_led.v