dmaengine: imx-dma: Check for clk_prepare_enable() error

clk_prepare_enable() may fail, so we should better check its return value
and propagate it in the case of error.

While at it, change the label 'err' to a more descriptive naming.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>