Fix dma when using copy.merge

[micprog]

Due to the type cast here:

pulp-sdk/rtos/pulpos/common/include/pos/implem/dma.h

Line 152 in aee8a8a

__cl_dma_memcpy(copy->ext, copy->loc, copy->size, copy->dir, copy->merge, (pi_cl_dma_cmd_t *)copy);

transfers are not properly handled when using the merge feature in the copy struct. This can cause stalls as the DMA is not properly executing transfers. This PR fixes part of the issue of the type cast, ensuring the ID field is the same in the cmd and copy struct. Other issues may come up when using the next field of the cmd struct.