tftp: Fix block buffer signedness warning

The buffer type differs in signedness (buffer is unsigned, local
pointer is signed), resulting in build warnings even with default
settings.

Correct the local type to fix the warnings.

Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>

tqftpserv.c

@@ -62,8 +62,8 @@ static ssize_t tftp_send_data(struct tftp_client *client,
 {
 	ssize_t len;
 	size_t send_len;
-	char *buf = client->blk_buf;
-	char *p = buf;
+	uint8_t *buf = client->blk_buf;
+	uint8_t *p = buf;
 
 	*p++ = 0;
 	*p++ = OP_DATA;