drivers/dma: fix bug that may copy one byte too much

This happens when src and dst are odd and len is even.

Example:

• dst = 0x13, src = 0x41, len = 14

Calculate length of DMA copy:

• -> dma0sz = len / 2 = 7

The first byte will be manually copied to word-align src and dst:

• -> dst = 0x14, src = 0x42, len = 13

Now the DMA transfer will copy 7 words (= 14 bytes), although only 13 bytes should be copied. This will therefore corrupt memory @ dst + len.