dma: coh901318: avoid unbalanced locking

In case the len is 0 we must return without trying to unlock the lock that was
not locked.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Vinod Koul <vinod.koul@intel.com>