dmaengine: at_xdmac: Remove unused inline function at_xdmac_csize()

commit 765c37d8

 ("dmaengine: at_xdmac: rework slave configuration part")
left behind this, so can remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20210407132543.23652-1-yuehaibing@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>