This commit refactors DAC driver. The functionality remains the same
but driver start up is now done in dac_setup (after application called
open function) instead of sam_dac_initialize (called from BSP). This
ensures that driver does not take resources (timer, interrupt) until
opened. Implementation of dac_shutdown is also provided, therefore
the driver frees resources once closed.
This change is consistent with other drivers implementation.
Signed-off-by: Michal Lenc <michallenc@seznam.cz>
fba4b2593c