cdc_ncm: return -ENOMEM if kzalloc fails

return -ENOMEM instead if kzalloc of cdc_ncm_ctx structure is failed.

and also remove the comparision of ctx structure with NULL and make
it as !ctx.

Signed-off-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>