mxc : Clean up properly when imx_dma_free() used without imx_dma_disable()

The sequence
	imx_dma_request()
	imx_dma_enable()
	imx_dma_free()
left the dma channel in_use mode and did not release the timer.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>