Convert cpu_to_le16(le16_to_cpu(frame->datalen) + len) to
use le16_add_cpu(), which is more concise and does the same thing.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>