net: cdc_ncm: use tasklet_init() for tasklet_struct init

The tasklet initialisation would be better done by tasklet_init()
instead of assuming all the fields are in an ok state by default.

This does not fix any actual know bug.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>