usb: device: fix the log level of a debug message

When using composite devices, an error is printed during the
initialization:

| [00:00:00.000,000] <err> usb_device: set cb, ep: 0x81
|
| [00:00:00.000,000] <err> usb_device: set cb, ep: 0x1
|
| [00:00:00.000,000] <err> usb_device: set cb, ep: 0x82
|
| [00:00:00.000,000] <err> usb_device: set cb, ep: 0x2

This is actually not an error, but rather a debug message. In addition
it should not contain a new line as it is automatically added by the
logger subsystem.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>