Hifiberry-DAC+:Avoids loading of headphone controls if not defined in DT-overlay

This fix keeps the driver from loading the headphone controls in case they are
not defined in the selected device tree overlay. This avoids problems if a
device uses the same I2C bus address as the headphone amp. The driver issues a
warning in that case.

Adds the missing embedded newlines in some printk/info statements.

Signed-off-by: Joerg Schambacher <joerg@hifiberry.com>