[fix][dma] fix index in dma2_isr for dma_callback

[vitfen]

Interrupt handeling for DMA2 on D0 ether crashes with 'Instruction access fault' or does not get handled at all. (Depends on the used bflb_device_s.idx value.)

With this patch this will be fixed. Also I had to reset the chip to recover and could than observe the expected behaviour.

For testing I used this device structure

struct bflb_device_s dev_desc_dma2_ch0 = {
    .name = "dma2_ch0",
    .reg_base = DMA2_BASE + 1 * DMA_CHANNEL_OFFSET,
    .irq_num = DMA2_INT0_IRQn,
    .idx = 2,
    .sub_idx = 0,
    .dev_type = BFLB_DEVICE_TYPE_DMA,
    .user_data = NULL
};

[sakumisue]

Thanks for your pr.