drivers: dma: fix the WCH DMA transfer width

The driver treats the `source_data_size` and `dest_data_size` as a
width in bits and converts 8 bits to 1, 16 bits to 2, and 32 bits to 3.

This should be a width in bytes with 1 byte mapping to 0, 2 bytes to
1, and 4 bytes to 3.

Note that this preserves the current behaviour of silently accepting
invalid transfer bit widths.

Signed-off-by: Michael Hope <michaelh@juju.nz>