AP_RangeFinder: Use common hexadecimal to character number conversion method

libraries/AP_RangeFinder/AP_RangeFinder_NMEA.cpp

@@ -134,7 +134,12 @@ bool AP_RangeFinder_NMEA::decode_latest_term()
 {
     // handle the last term in a message
     if (_term_is_checksum) {
-        uint8_t checksum = 16 * char_to_hex(_term[0]) + char_to_hex(_term[1]);
+        uint8_t nibble_high = 0;
+        uint8_t nibble_low  = 0;
+        if (!hex_to_uint8(_term[0], nibble_high) || !hex_to_uint8(_term[1], nibble_low)) {
+            return false;
+        }
+        const uint8_t checksum = (nibble_high << 4u) | nibble_low;
         return ((checksum == _checksum) &&
                 !is_negative(_distance_m) &&
                 (_sentence_type == SONAR_DBT || _sentence_type == SONAR_DPT));
@@ -174,14 +179,3 @@ bool AP_RangeFinder_NMEA::decode_latest_term()
 
     return false;
 }
-
-// return the numeric value of an ascii hex character
-int16_t AP_RangeFinder_NMEA::char_to_hex(char a)
-{
-    if (a >= 'A' && a <= 'F')
-        return a - 'A' + 10;
-    else if (a >= 'a' && a <= 'f')
-        return a - 'a' + 10;
-    else
-        return a - '0';
-}

libraries/AP_RangeFinder/AP_RangeFinder_NMEA.h

@@ -60,9 +60,6 @@ private:
     // returns true if new sentence has just passed checksum test and is validated
     bool decode_latest_term();
 
-    // return the numeric value of an ascii hex character
-    static int16_t char_to_hex(char a);
-
     AP_HAL::UARTDriver *uart = nullptr;     // pointer to serial uart
 
     // message decoding related members