dmaengine: pl330: report BURST residue granularity

The reported residue is already calculated in BURST unit granularity, so
advertise this capability properly to other devices in the system.

Fixes: aee4d1fa ("dmaengine: pl330: improve pl330_tx_status() function")
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>