usb: gadget: u_ether: Check tx_reqs before allocating new SKB

Commit fdafb970 ("usb: gadget: u_ether: reorganize code for
better readability") refactored the eth_start_xmit function to
free old skb first, add RNDIS header to new SKB and then check
if disconnect interrupt has freed up the tx_reqs. If this
happens, then on next eth_start_xmit we will end up using the
previous skb which leads to page fault.
Fix this by first checking tx_reqs availability and then adding
RNDIS header.

Change-Id: I0bf59361c71c989a436685917301cabd62b91e95
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>