All USB Ethernet functions will have very similar attributes in configfs.

This patch provides helper definitions to ease writing the functions and
reduce source code duplication.
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>