drivers: dma_xmc4xxx: Support calling dma_stop() after dma_config()

In the dma loop_transfer test (test_loop_repeated_start_stop()), the
order of calls is dma_config() -> dma_stop() -> dma_start().  This
currently does not work on xmc4xxx because after calling dma_stop(), the
dma would be in a suspended state. Fix this by leaving the suspended state
before exiting dma_stop().

Also don't clear the context (dma_channel->dlr_line and dma_channel->cb)
that was setup by dma_config().

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