llc: gprs_llc_fcs(): make the input data pointer const

Change-Id: If95210fe69f915ab1010fe5916cb6bee3faeb7b9
diff --git a/include/osmocom/sgsn/crc24.h b/include/osmocom/sgsn/crc24.h
index 756638c..c913eaf 100644
--- a/include/osmocom/sgsn/crc24.h
+++ b/include/osmocom/sgsn/crc24.h
@@ -5,6 +5,6 @@
 
 #define INIT_CRC24	0xffffff
 
-uint32_t crc24_calc(uint32_t fcs, uint8_t *cp, unsigned int len);
+uint32_t crc24_calc(uint32_t fcs, const uint8_t *cp, unsigned int len);
 
 #endif
diff --git a/include/osmocom/sgsn/gprs_llc.h b/include/osmocom/sgsn/gprs_llc.h
index 2c6c98b..6cdb995 100644
--- a/include/osmocom/sgsn/gprs_llc.h
+++ b/include/osmocom/sgsn/gprs_llc.h
@@ -277,7 +277,7 @@
 int gprs_llc_hdr_parse(struct gprs_llc_hdr_parsed *ghp,
 		       uint8_t *llc_hdr, int len);
 void gprs_llc_hdr_dump(struct gprs_llc_hdr_parsed *gph, struct gprs_llc_lle *lle);
-int gprs_llc_fcs(uint8_t *data, unsigned int len);
+int gprs_llc_fcs(const uint8_t *data, unsigned int len);
 
 
 /* LLME handling routines */