usb: gadget: f_ncm: remove timer_force_tx field

usb: gadget: f_ncm: remove timer_force_tx field

It is simply not needed.  This field is equivalent to skb being NULL.

Currently with the boolean set to true we call:
  ncm->netdev->netdev_ops->ndo_start_xmit(NULL, ncm->netdev);
which calls u_ether's:
  eth_start_xmit(NULL, ...)
which then calls:
  skb = dev->wrap(dev->port_usb, NULL);
which calls back into f_ncm's:
  ncm_wrap_ntb(..., NULL)

Cc: Brooke Basile <brookebasile@gmail.com>
Cc: "Bryan O'Donoghue" <bryan.odonoghue@linaro.org>
Cc: Felipe Balbi <balbi@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Link: https://lore.kernel.org/r/20210701114834.884597-1-zenczykowski@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>