(powerpc/512) Fix possible `dma_unmap_single()` on uninitialized pointer

If the device configuration fails (if `dma_dev->device_config()`),
`sg_dma_address(&sg)` is not initialized and the jump to `err_dma_prep`
leads to calling `dma_unmap_single()` on `sg_dma_address(&sg)`.

Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250610142918.169540-2-fourier.thomas@gmail.com