hw/dma: sifive_pdma: reset Next* registers when Control.claim is set

Setting Control.claim clears all of the chanel's Next registers.
This is effective only when Control.claim is set from 0 to 1.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Tested-by: Max Hsu <max.hsu@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210912130553.179501-2-frank.chang@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>