Some CoreSight components have trace bus clocks 'atclk' and are enabled
using clk_prepare_enable(). These clocks are not disabled when modules
exit.
As atclk is optional, use devm_clk_get_optional_enabled() to manage it.
The benefit is the driver model layer can automatically disable and
release clocks.
Check the returned value with IS_ERR() to detect errors but leave the
NULL pointer case if the clock is not found. And remove the error
handling codes which are no longer needed.
Fixes: d1839e687773 ("coresight: etm: retrieve and handle atclk")