greatfet: don't print misleading "unknown" libusb error text

Only lookup libusb error messages for negative return values which
communicate error codes. Positive return values are transmission counts
and are not an error.

The "UNKNOWN" messages only were seen at higher log levels. Users were
not affected by the issue.

Author: gsigh

src/hardware/greatfet/protocol.c

@@ -169,7 +169,7 @@ static int greatfet_ctrl_out_in(const struct sr_dev_inst *sdi,
 		flags, (void *)tx_data, tx_size, timeout_ms);
 	if (sr_log_loglevel_get() >= SR_LOG_SPEW) {
 		const char *msg;
-		msg = ret ? libusb_error_name(ret) : "-";
+		msg = ret < 0 ? libusb_error_name(ret) : "-";
 		sr_spew("USB out, rc %d, %s", ret, msg);
 	}
 	if (ret < 0) {
@@ -197,7 +197,7 @@ static int greatfet_ctrl_out_in(const struct sr_dev_inst *sdi,
 		0, rx_data, rx_size, timeout_ms);
 	if (sr_log_loglevel_get() >= SR_LOG_SPEW) {
 		const char *msg;
-		msg = ret ? libusb_error_name(ret) : "-";
+		msg = ret < 0 ? libusb_error_name(ret) : "-";
 		sr_spew("USB in, rc %d, %s", ret, msg);
 	}
 	if (ret < 0) {