drivers: net: xgene: fix possible use after free

Once TX has been enabled on a NIC, it is illegal to access skb,
as this skb might have been freed by another cpu, from TX completion
handler.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Cc: Iyappan Subramanian <isubramanian@apm.com>
Acked-by: Iyappan Subramanian <isubramanian@apm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>