ata: sata_dwc_460ex: burst size must be in items not bytes

The burst size as defined by DMAengine API is in items of address width. Derive
burst size from AHB_DMA_BRST_DFLT (64 bytes) by dividing it to
DMA_SLAVE_BUSWIDTH_4_BYTES (4 bytes) that gives us 16 items.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>