Fix DNS resolution errors on AP reconnection

[sandeepmistry]

This change in combination with #12, seems to resolve arduino-libraries/WiFiNINA#49 for me.

@Rocketct could you please test?

[sandeepmistry]

@Rocketct can you please re-test this, I've the issue and have better address it. When the issue occurs the DNS server of the module is 0.0.0.0. This was happening in 2 conditions:

1. We thought a static IP was assigned, because _ipInfo.ip.addr was set with a value from the previous DHCP release, so the DNS servers were set to 0.0.0.0. In d458b90 I've added a separate variable to track if a static IP is configured.

2. Calling tcpip_adapter_dhcpc_start(TCPIP_ADAPTER_IF_STA) were sometimes also clear the DNS servers, in 04ec799 this has been removed.

[Rocketct]

Tested with a Uno wifi rev 2 the fix works fine!! the boards is able to resolve the DNS each time that i disconnect the AP and is able to reconnect to the server domain.