dmaengine: fsl-edma: fix edma4 channel enable failure on second attempt

When attempting to start DMA for the second time using
fsl_edma3_enable_request(), channel never start.

CHn_MUX must have a unique value when selecting a peripheral slot in the
channel mux configuration. The only value that may overlap is source 0.
If there is an attempt to write a mux configuration value that is already
consumed by another channel, a mux configuration of 0 (SRC = 0) will be
written.

Check CHn_MUX before writing in fsl_edma3_enable_request().

Fixes: 72f5801a

 ("dmaengine: fsl-edma: integrate v3 support")
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20230823182635.2618118-1-Frank.Li@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>