lib: os: fix signed and unsigend comparison warnings

Fix instances of:
warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>
diff --git a/lib/os/crc8_sw.c b/lib/os/crc8_sw.c
index 8c6d32c..eb052c9 100644
--- a/lib/os/crc8_sw.c
+++ b/lib/os/crc8_sw.c
@@ -15,7 +15,7 @@
 
 u8_t crc8_ccitt(u8_t val, const void *buf, size_t cnt)
 {
-	int i;
+	size_t i;
 	const u8_t *p = buf;
 
 	for (i = 0; i < cnt; i++) {