AP_Math: make crc_crc8 const

libraries/AP_Math/crc.cpp

@@ -47,7 +47,7 @@ static const uint8_t crc8_table[] = {
 /*
   crc8 from trone driver by Luis Rodrigues
  */
-uint8_t crc_crc8(uint8_t *p, uint8_t len)
+uint8_t crc_crc8(const uint8_t *p, uint8_t len)
 {
 	uint16_t i;
 	uint16_t crc = 0x0;

libraries/AP_Math/crc.h

@@ -16,5 +16,5 @@
   interfaces to ArduPilot collection of CRCs. 
  */
 
-uint8_t crc_crc8(uint8_t *p, uint8_t len);
+uint8_t crc_crc8(const uint8_t *p, uint8_t len);