net/tftp.c: fix warning: pointer targets differ in signedness

tftp.c:294: warning: pointer targets in passing argument 1 of 'strlen'
differ in signedness

This was only visible for the utx8245 board which seems to have DEBUG
enabled.

Signed-off-by: Wolfgang Denk <wd@denx.de>

net/tftp.c

@@ -291,7 +291,9 @@ TftpHandler (uchar * pkt, unsigned dest, unsigned src, unsigned len)
 		break;
 
 	case TFTP_OACK:
-		debug("Got OACK: %s %s\n", pkt, pkt+strlen(pkt)+1);
+		debug("Got OACK: %s %s\n",
+			pkt,
+			pkt + strlen((char *)pkt) + 1);
 		TftpState = STATE_OACK;
 		TftpServerPort = src;
 		/*