Fixed IPs of dhcp_port are not updated when a new subnet is added to the network.

Observations:
Create a Neutron tenant network (n1).
Create an IPv4 subnet with 10.0.0.0/24 CIDR in network (n1).
Create one more IPv4 subnet with 20.0.0.0/24 subnet in the same network
(n1).
DHCP port in Neutron db has two fixed ips (i.e., 10.0.0.2 and 20.0.0.2),
whereas md-sal db has only one fixed_ip (i.e., 10.0.0.2).

Fixed_ips of dhcp_port are not updated when multiple subnets are added to a network.