AP_Compass: removed assert calls

these waste flash space and do not do us any good

libraries/AP_Compass/AP_Compass_AK09916.cpp

@@ -391,7 +391,9 @@ bool AP_AK09916_BusDriver_Auxiliary::block_read(uint8_t reg, uint8_t *buf, uint3
          * We can only read a block when reading the block of sample values -
          * calling with any other value is a mistake
          */
-        assert(reg == REG_ST1);
+        if (reg != REG_ST1) {
+            return false;
+        }
 
         int n = _slave->read(buf);
         return n == static_cast<int>(size);

libraries/AP_Compass/AP_Compass_AK8963.cpp

@@ -328,7 +328,9 @@ bool AP_AK8963_BusDriver_Auxiliary::block_read(uint8_t reg, uint8_t *buf, uint32
          * We can only read a block when reading the block of sample values -
          * calling with any other value is a mistake
          */
-        assert(reg == AK8963_HXL);
+        if (reg != AK8963_HXL) {
+            return false;
+        }
 
         int n = _slave->read(buf);
         return n == static_cast<int>(size);

libraries/AP_Compass/AP_Compass_HMC5843.cpp

@@ -521,7 +521,9 @@ bool AP_HMC5843_BusDriver_Auxiliary::block_read(uint8_t reg, uint8_t *buf, uint3
          * We can only read a block when reading the block of sample values -
          * calling with any other value is a mistake
          */
-        assert(reg == HMC5843_REG_DATA_OUTPUT_X_MSB);
+        if (reg != HMC5843_REG_DATA_OUTPUT_X_MSB) {
+            return false;
+        }
 
         int n = _slave->read(buf);
         return n == static_cast<int>(size);