ACPI: EC: Simplify acpi_ec_ecdt_start() and acpi_ec_init()

Notice that the return value of acpi_ec_init() is discarded anyway,
so it can be void and it doesn't need to check the return values of
acpi_bus_register_driver() and acpi_ec_ecdt_start() called by it.
Thus the latter can be void too and it really has nothing to do
if acpi_ec_add() has already found an EC matching the boot one in the
namespace.  Also, acpi_ec_ecdt_get_handle() can be folded into it.

Modify the code accordingly and while at it create a propoer kerneldoc
comment to document acpi_ec_ecdt_start() and move the remark regarding
ASUS X550ZE along with the related bug URL from acpi_ec_init() into
that comment.

Additionally, fix up a stale comment in acpi_ec_init().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>