net: fix check for number of parameters to -netdev socket

Since commit 0f8c289ad "net: fix -netdev socket,fd= for UDP sockets"
we allow more than one parameter for -netdev socket. But now
we run into an assert when no parameter at all is specified

> qemu-system-x86_64 -netdev socket
socket.c:729: net_init_socket: Assertion `sock->has_udp' failed.

Fix this by reverting the change of the if condition done in 0f8c289ad.

Cc: Jason Wang <jasowang@redhat.com>
Cc: qemu-stable@nongnu.org
Fixes: 0f8c289ad539feb5135c545bea947b310a893f4b
Reported-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
