usb: gadget: udc: atmel: remove memory leak

Commit bbe097f0

 ("usb: gadget: udc: atmel: fix endpoint name")
introduced a memory leak when unbinding the driver. The endpoint names
would not be freed. Solve that by including the name as a string in struct
usba_ep so it is freed when the endpoint is.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>