usb: gadget: add helpers for configfs support for USB Ethernet

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>