dmaengine: dw: fix regression in dw_probe() function

The commit dbde5c29

 "dw_dmac: use devm_* functions to simplify code" turns
probe function to use devm_* helpers and simultaneously brings a regression.

We have to 1) call clk_disable_unprepare() on error path, and 2) check error
code of clk_enable_prepare(). First part was done in the original code, second
one is an update.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>