net: usb: pegasus: fixes of set_register(s) return value evaluation;

- restore the behavior in enable_net_traffic() to avoid regressions - Jakub
    Kicinski;
  - hurried up and removed redundant assignment in pegasus_open() before yet
    another checker complains;

Fixes: 8a160e2e ("net: usb: pegasus: Check the return value of get_geristers() and friends;")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Petko Manolov <petko.manolov@konsulko.com>
Signed-off-by: David S. Miller <davem@davemloft.net>