dmaengine: imx-sdma: remove useless 'lock' and 'enabled' in 'struct sdma_channel'

Since 'sdmac->vc.lock' and 'sdmac->desc' can be used as 'lock' and
'enabled' in 'struct sdma_channel sdmac', remove them.

Signed-off-by: Robin Gong <yibin.gong@nxp.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Vinod Koul <vkoul@kernel.org>