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.

CRs-fixed: 595367
Change-Id: I181da831120b38840ae4b8b74c1dacce129d1ba6
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>