usb: gadget: eem: fix echo command processing

During processing of bunch of eem frames if "echo" command is found
skb is cloned and the cloned version should be used to send reply.
Unfortunately, the data of the original skb were actually used and
the cloned skb is never freed.

Using the cloned skb and freeing the skb in the completion callback
for usb request.

Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@nokia.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>