usb: gadget: ether: Add \n to each attribute of ethernet functions

Generally in SysFS and ConfigFS files are new line terminated.
Also most of USB functions adds a trailing newline to each attribute.
Let's follow this convention also in ethernet functions.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>