dmaengine: set_dma40: balance clock in probe fail code

Clock code was changed to use clk_prepare_enable in:

b707c65 dma/ste_dma40: Fixup clock usage during probe

but clk_disable on probe fail path was not updated.  This patch fix this
by using clk_disable_unprepare in place of clk_disable.

Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>