rm_freertos_plus_tcp: FreeRTOS_IsNetworkUp returns true before DHCP finishes

[SeanMeng0509]

Issue

FreeRTOS_IsNetworkUp will return true on Phy link up event. When DHCP is enabled, device won't have a valid IP address at that time, which could lead to subsequent socket operation failure.

Workaround

Instead of calling FreeRTOS_IsNetworkUp, applications shall check whether FreeRTOS_GetIPAddress() returns a valid DHCP address before any socket operation.

[SeanMeng0509]

This is being internally tracked using RAP-2935.