ALSA: atmel_abdac: Add missing clock prepare

Clocks must be prepared before enabling them. Do this in one step.
Replace clk_enable with clk_prepare_enable and clk_disable with
clk_disable_unprepare.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>