net: nb8800: Fix SKB leak in nb8800_receive()

In case nb8800_receive() fails to allocate a fragment, we would leak the
SKB freshly allocated and just return, instead, free it.

Reported-by: coverity (CID 1341750)
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: David S. Miller <davem@davemloft.net>