usb: gadget: ether: Add counters for tx packets

This counter represents number of packets recevied from n/w stack and
will help debug packet drop issues. Also count actual data transferred
instead of length.

Change-Id: I6bbd3b4759cde075d48228d40052de00e20796c4
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>