riscv: k210: Reduce DMA block size

Although the max DMA size supported by the hardware register is 22 bits (or
4M), the Linux driver for this device only supports transfers of up to 2M.
On a device with 8M total memory, I don't think it will make too much of a
difference.

Reported-by: Damien Le Moal <Damien.LeMoal@wdc.com>
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>