dmaengine: at_xdmac: move spin_lock_bh to spin_lock in tasklet

as you are already in a tasklet, it is unnecessary to call spin_lock_bh.

Signed-off-by: Barry Song <21cnbao@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>