net: cdc_ncm: use DEVICE_ATTR_RW macro

Use DEVICE_ATTR_RW helper instead of plain DEVICE_ATTR,
which makes the code a bit shorter and easier to read.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>