dmaengine: at_hdmac: prepare clk before calling enable

Replace clk_enable/disable with clk_prepare_enable/disable_unprepare to
avoid common clk framework warnings.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
[nicolas.ferre@atmel.com: remove return code checking in at_dma_resume_noirq()]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>