net: usb: cdc_ncm: use new API for bh tasklet

This converts the driver to use the new tasklet API introduced in
commit 12cc923f

 ("tasklet: Introduce new initialization API")

It is unfortunate that we need to add a pointer to the driver context to
get back to the usbnet device, but the space will be reclaimed once
there are no more users of the old API left and we can remove the data
value and flag from the tasklet struct.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20210130234637.26505-1-kernel@esmil.dk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>