drivers: dma_xmc4xxx: Clear request before callback when overrun happens

If an overrun happens the user may decide to stop the dma which
unsets the DMA line. Currently, the line is always re-enabled after the
callback.

Switch the order around to fix the problem. Also, always reset the line
even if the user doesn't provide a callback.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>