chan_mobile: decrease CHANNEL_FRAME_SIZE to prevent delay

On modern Bluetooth devices or lower-powered asterisk servers, decreasing the channel frame size significantly improves latency and delay on outbound calls with only a mild sacrifice to the quality of the call (the frame size before was massive overkill to begin with)

(cherry picked from commit 5840e55bb5)

addons/chan_mobile.c

@@ -80,7 +80,7 @@
 
 #define DEVICE_FRAME_SIZE 48
 #define DEVICE_FRAME_FORMAT ast_format_slin
-#define CHANNEL_FRAME_SIZE 320
+#define CHANNEL_FRAME_SIZE 80
 
 static int discovery_interval = 60;			/* The device discovery interval, default 60 seconds. */
 static pthread_t discovery_thread = AST_PTHREADT_NULL;	/* The discovery thread */