xdp: move the if dev statements to the first

The xdp_rxq_info_unreg() called by xdp_rxq_info_reg() is meaningless when
dev is NULL, so move the if dev statements to the first.

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>