drivers: net: fix return value check in emac_tso_csum()

in emac_tso_csum(), return an error code if an unexpected value
is returned by pskb_trim().

Signed-off-by: Yuanjun Gong <ruc_gongyuanjun@163.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>