trx/udp_link.py: close socket in destructor

Previously it was required to call the UDPLink.shutdown() method
manually in order to close a socket. Let's do it automatically
using the destructor of UDPLink.

Cherry-picked from: I59c3dc61ec58cd9effeb789947d28fd602ca91f4
Change-Id: Ief7aa21d1e50682a90616833b679741957193aae

apps/grgsm_trx

@@ -83,7 +83,6 @@ class Application:
 
 	def shutdown(self):
 		print("[i] Shutting down...")
-		self.server.shutdown()
 		self.radio.shutdown()
 
 	def print_copyright(self):

python/trx/ctrl_if_bb.py

@@ -35,10 +35,6 @@ class ctrl_if_bb(ctrl_if):
 		# Power measurement
 		self.pm = pm
 
-	def shutdown(self):
-		print("[i] Shutdown CTRL interface")
-		ctrl_if.shutdown(self)
-
 	def parse_cmd(self, request):
 		# Power control
 		if self.verify_cmd(request, "POWERON", 0):

python/trx/udp_link.py

@@ -35,6 +35,9 @@ class udp_link:
 		self.remote_addr = remote_addr
 		self.remote_port = remote_port
 
+	def __del__(self):
+		self.sock.close()
+
 	def loop(self):
 		r_event, w_event, x_event = select.select([self.sock], [], [])
 
@@ -43,9 +46,6 @@ class udp_link:
 			data, addr = self.sock.recvfrom(128)
 			self.handle_rx(data.decode())
 
-	def shutdown(self):
-		self.sock.close();
-
 	def send(self, data):
 		if type(data) not in [bytearray, bytes]:
 			data = data.encode()