dmaengine: at_xdmac: fix software lockup at_xdmac_tx_status()

According to the Atmel eXtended DMA controller datasheet, requesting a
DMA transfer flush for a channel is only revelant when this transfer is
source peripheral synchronized.

So we have to check this condition before requesting a channel flush by
writing the channel bit into the Global channel SoftWare Flush (GSWF)
register then waiting for flush to complete by monitoring the end of
Flush Interrupt Status (FIS) bit in the Channel Interrupt Status (CIS)
register.

Indeed, for non source peripheral synchronized transfer, writing the
channel bit into the GSWF register does nothing. Especially, the FIS bit
is never set into the CIS register. The former code looped forever
waiting for this bit to be set.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>