USB: gadget: ether: Fix memory corruption issues with multi_pkt_xfers

If gadget xmits packets before host sends RNDIS_MSG_INIT to
update maximum packets supported per transfer, then there is a
possibility of using freed skb memory later from xmit function.
Fix such races which may arise between tx_complete and start_xmit
routines depending on when multi_pkt_transfers are enabled. Other
option is to defer starting TX transfers until RNDIS_MSG_INIT is
received, but then it makes it difficult to handle scenarios
where host doesn't support this message or just doesn't send it.

Change-Id: I181da831120b38840ae4b8b74c1dacce129d1ba6
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>