UPSTREAM: netpoll: Fix device name check in netpoll_setup()

Apparently netpoll_setup() assumes that netpoll.dev_name is a pointer
when checking if the device name is set:

if (np->dev_name) {
  ...

However the field is a character array, therefore the condition always
yields true. Check instead whether the first byte of the array has a
non-zero value.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 0c3a8f8b8fabff4f3ad2dd7b95ae0e90cdd1aebb)
Bug: 78886293
Change-Id: I1a6eec091c4bab5769a3519196f529030a71b6dd
Signed-off-by: Alistair Strachan <astrachan@google.com>