usb: gadget: u_ether: eliminate abuse of ep->driver data

Since ep->driver_data is not used for endpoint claiming, neither for
enabled/disabled state storing, we can reduce number of places where
we read or modify it's value, as now it has no particular meaning for
function or framework logic.

In case of u_ether we only need to store in ep->driver_data pointer to
struct eth_dev, as it's used in rx_complete() and tx_complete() callbacks.
All other uses of ep->driver_data are now meaningless and can be safely
removed.

Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>