net: cdc_eem: fix tx fixup skb leak

In the Linux kernel, the following vulnerability has been resolved:

net: cdc_eem: fix tx fixup skb leak

when usbnet transmit a skb, eem fixup it in eem_tx_fixup(),

if skb_copy_expand() failed, it return NULL,

usbnet_start_xmit() will have no chance to free original skb.

fix it by free orginal skb in eem_tx_fixup() first,

then check skb clone status, if failed, return NULL to usbnet.