ethernet: atheros: fix return value check in atl1c_tso_csum()

in atl1c_tso_csum, it should check the return value of pskb_trim(),
and return an error code if an unexpected value is returned
by pskb_trim().

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>