axi_dmac: data_mover: Improve timing

The pending_burst signal and the expression id != request_id are almost
identical. pending_burst goes high with a delay of one clock cycle, but the
important thing is that it goes low on the same clock cycle as the expression.
By using pending_burst here instead of 'id != request_id' we can reduce the
fanout of the 'id' register and improve the timing of the core.