dbdma: fix endian of DBDMA_CMDPTR_LO during branch

The current DBDMA command is stored in little-endian format, so make sure
we convert it to match our CPU when updating the DBDMA_CMDPTR_LO register.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>