dmaengine: fsl-edma: clear pending interrupts on initialization

Clear pending interrupts before requesting interrupts and move
interrupt initialization after channels have been initialized.
This avoids a NULL pointer dereference panic when using kexec
while DMA requests were running.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>