sm8450-common: gps: Implement list empty error code for msg_q

This fixes enum-conversion warning due to implicit type enum casting
between linked_list and msg_q which happens only for status -6.

Change-Id: I27ba74c4d840b7ba0cb1017d69a30fc25b80d6b9

gps/utils/msg_q.h

@@ -50,6 +50,8 @@ typedef enum
      /**< Failed because an there were not enough resources. */
   eMSG_Q_INSUFFICIENT_BUFFER                 = -5,
      /**< Failed because an the supplied buffer was too small. */
+  eMSG_Q_EMPTY                               = -6
+     /**< Failed because list is empty. */
 }msq_q_err_type;
 
 /*===========================================================================