xdp: fix uninitialized 'err' variable

Smatch caught an uninitialized variable error which GCC seems
to miss.

Fixes: a25717d2 ("xdp: support simultaneous driver and hw XDP attachment")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>