usbnet: fix status interrupt urb handling

Since commit 7b0c5f21
"sierra_net: keep status interrupt URB active", sierra_net triggers
status interrupt polling before the net_device is opened (in order to
properly receive the sync message response).

To be able to receive further interrupts, the interrupt urb needs to be
re-submitted, so this patch removes the bogus check for netif_running().

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>