Change return type of slip_send to void

The return value was not checked anywhere in the code

os/dev/slip.c

@@ -84,7 +84,7 @@ slip_set_input_callback(void (*c)(void))
   input_callback = c;
 }
 /*---------------------------------------------------------------------------*/
-uint8_t
+void
 slip_send(void)
 {
   uint16_t i;

os/dev/slip.h

@@ -41,7 +41,7 @@ PROCESS_NAME(slip_process);
 /**
  * Send an IP packet from the uIP buffer with SLIP.
  */
-uint8_t slip_send(void);
+void slip_send(void);
 
 /**
  * Input a SLIP byte.