net: stats: Fix UDP packet counter for DHCPv4 TX

Fix UDP TX packet count when DHCPv4 is used. Currently UDP TX counter
is only updated when net context is in use. DHCPv4 however does not use
net_context underneath, therefore it needs to update UDP TX stats on its
own.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>