dmaengine/ste_dma40: clear LNK on channel startup

Otherwise if a previously physical channel is used as a logical channel, the
LNK may have old values which affect the operation of the logical channel since
the LNK register has different semantics between physical and logical channels.

Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Signed-off-by: Narayanan G <narayanan.gopalakrishnan@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>